The K-3 will do fine up to 3200 with proper noise reduction took me a while after the K-5 to figure out best way to post process and thought I had made a mistake upgrading to K-3 but after a while I figured out it just seemed to have more noise but it was a noise that was easily removed.

This was shot at ISO8000
IMGP4895-1 by Tim Crumly, on Flickr

I cannot compare with or comment about the Pentax K3, because I've never used one.

I also haven't used the T1i, but I have used other 15MP Canon cameras (50D) and can tell you that just about any of the current models will be considerably higher ISO capable. The 20MP and 24MP Canon, in particular those with the Digic 6 or Digic 7 processors, are very capable of high ISO.

The image below was done with one of my 7D Mark II, a test shot at ISO 16000 (not a type.... sixteen thousand... not 1600). It helps that image noise can "hide" in images of furry critters... But you'll see some noise in the enlarged crop. However I think it's pretty well controlled. I've sold images that were shot with these cameras at ISO 6400, 12800 and even occasionally 16000. I shoot RAW and am careful to avoid underexposure, sometimes even over-expose a little and "pull back" the image in post processing. The attached image has no in-camera NR applied, RAW conversion was done in Lightroom using only the default level of NR there. When I'm finishing a high ISO image more fully, I'll use Imagenomic Noiseware plug-in in Photoshop to further reduce what's seen in this image.



All the current Canon APS-C models - both DSLR and mirrorless M-series - use the even newer 24MP sensor, except for the 7DII with 20MP (same as in 70D, I think). Of them all, the only one I would be concerned about is the most entry-level T7 model (no "i"), which uses a Digic 4 processor, same as the model it replaced (also the same processor as in your T1i). All the others use Digic 6, Digic 7 or in a few cases now, Digic 8. (Note, the latter now have a new RAW file format.... CR3, as opposed to CR2 found in all the earlier). In addition to sensor improvements, the newer and more powerful processors contribute to image quality and noise handling, too.

It might help put it in perspective, when I was shooting with 15MP 50D APS-C cameras, I used a 21MP full frame 5D Mark II alongside them especially for high ISO purposes (among other things), up to 6400. I continued to use it alongside 18MP 7D original model cameras, when I was shooting with those for about five years. But since getting 20MP 7D Mark II a couple years ago, I hardly use the 5DII at all. Never for high ISO work. The 7DIIs handle that just as well or better. Sure, I could update to a newer version of full frame to have even higher ISO.... but it's pretty rare I need higher than 6400 or, perhaps, 12800. And the 7DIIs handle that as well as or better than the 5DII. Below is an ISO 6400 test shot done with 5DII (and the EF 135mm f/2L I bought with it)...



You might notice a theme in my test shots!

In the end, I think you would be happy with almost any of the current Canon APS-C cameras.... and could continue to use the lenses and perhaps more of the accessories you have now (flashes, memory cards, etc.) I think you'd get the most "bang for your buck" with 80D or 77D.... but if you prefer the Rebel-style controls, the T7i or smaller SL2 might be worth consideration. Any of those models is within your budget. Some of them would leave a lot left over for other things! Of those, the SL2 has an AF system very similar to your camera's, while the other three use the more advanced 45-point AF system that was introduced in the 80D (and is able to work in lower light conditions too). They all have articulated Touch Screen LCDs and use Canon's Dual Pixel AF in Live View (a huge improvement over the old contrast detection AF in Live View on cameras prior to the 70D). The 80D also has 100% viewfinder using a true pentaprism (bigger & brighter), Micro Focus Adjustment, and a higher specification shutter (top speed 1/8000 & 1/250 flash sync, also rated for a longer life span).

I wouldn't rule out the mirrorless M-series cameras either, if those interest you. The M5 and the newer M50 have electronic viewfinders, while the M6 and M100 don't (I think the M100 is of the newest models using the Digic 8 processor, with the new CR3 RAW file format). Any EF/EF-S lenses you have can be used on the M-series with an adapter.

Have fun shopping!

I cannot compare with or comment about the Pentax K3, because I've never used one.

I also haven't used the T1i, but I have used other 15MP Canon cameras (50D) and can tell you that just about any of the current models will be considerably higher ISO capable. The 20MP and 24MP Canon, in particular those with the Digic 6 or Digic 7 processors, are very capable of high ISO.

The image below was done with one of my 7D Mark II, a test shot at ISO 16000 (not a type.... sixteen thousand... not 1600). It helps that image noise can "hide" in images of furry critters... But you'll see some noise in the enlarged crop. However I think it's pretty well controlled. I've sold images that were shot with these cameras at ISO 6400, 12800 and even occasionally 16000. I shoot RAW and am careful to avoid underexposure, sometimes even over-expose a little and "pull back" the image in post processing. The attached image has no in-camera NR applied, RAW conversion was done in Lightroom using only the default level of NR there. When I'm finishing a high ISO image more fully, I'll use Imagenomic Noiseware plug-in in Photoshop to further reduce what's seen in this image.



All the current Canon APS-C models - both DSLR and mirrorless M-series - use the even newer 24MP sensor, except for the 7DII with 20MP (same as in 70D, I think). Of them all, the only one I would be concerned about is the most entry-level T7 model (no "i"), which uses a Digic 4 processor, same as the model it replaced (also the same processor as in your T1i). All the others use Digic 6, Digic 7 or in a few cases now, Digic 8. (Note, the latter now have a new RAW file format.... CR3, as opposed to CR2 found in all the earlier). In addition to sensor improvements, the newer and more powerful processors contribute to image quality and noise handling, too.

It might help put it in perspective, when I was shooting with 15MP 50D APS-C cameras, I used a 21MP full frame 5D Mark II alongside them especially for high ISO purposes (among other things), up to 6400. I continued to use it alongside 18MP 7D original model cameras, when I was shooting with those for about five years. But since getting 20MP 7D Mark II a couple years ago, I hardly use the 5DII at all. Never for high ISO work. The 7DIIs handle that just as well or better. Sure, I could update to a newer version of full frame to have even higher ISO.... but it's pretty rare I need higher than 6400 or, perhaps, 12800. And the 7DIIs handle that as well as or better than the 5DII. Below is an ISO 6400 test shot done with 5DII (and the EF 135mm f/2L I bought with it)...



You might notice a theme in my test shots!

In the end, I think you would be happy with almost any of the current Canon APS-C cameras.... and could continue to use the lenses and perhaps more of the accessories you have now (flashes, memory cards, etc.) I think you'd get the most "bang for your buck" with 80D or 77D.... but if you prefer the Rebel-style controls, the T7i or smaller SL2 might be worth consideration. Any of those models is within your budget. Some of them would leave a lot left over for other things! Of those, the SL2 has an AF system very similar to your camera's, while the other three use the more advanced 45-point AF system that was introduced in the 80D (and is able to work in lower light conditions too). They all have articulated Touch Screen LCDs and use Canon's Dual Pixel AF in Live View (a huge improvement over the old contrast detection AF in Live View on cameras prior to the 70D). The 80D also has 100% viewfinder using a true pentaprism (bigger & brighter), Micro Focus Adjustment, and a higher specification shutter (top speed 1/8000 & 1/250 flash sync, also rated for a longer life span).

I wouldn't rule out the mirrorless M-series cameras either, if those interest you. The M5 and the newer M50 have electronic viewfinders, while the M6 and M100 don't (I think the M100 is of the newest models using the Digic 8 processor, with the new CR3 RAW file format). Any EF/EF-S lenses you have can be used on the M-series with an adapter.

Have fun shopping!

Interesting question to which I personally don't have an answer, just an opinion. When shooting night football, which I just completed for the year, I use the D5 with a 400/2.8 and the D850 with a 70-200/2.8. I am happy with the results of both but I think I prefer the D5 images just a smidgeon. As a former D810 owner/user I think the low light performance of that camera was better than that of the D850, but again, by a small measure. I switched to the D850 not because I was unhappy with the D810, but because I found it useless for sports action, mainly due to its buffer. None of what I said is scientific or fact based, it just my musings. I find the D500 a damn good camera, especially for the price. Pair it with a 300/4 PF and you've got one powerful, lightweight monster! The D500 doesn't get close to the others in low light performance. Best of luck.