Depends on your photography needs and wallet:
- D750 is better for landscapes, art, fashion and low-light photography as it has more dynamic range and produces less noise.
- D500 is better for fast-moving sports and wildlife as it has probably the best AF acquisition and tracking system on the market; plus it's a cropped camera putting more pixels on an animal at long distances.
- D500 has an extremely fast 10 fps and a deep buffer to store 200 14-bit RAW images; D750 does 6.5 fps and has a RAW image buffer of about 21 14-bit images - sounds like an impressive difference but do you really need it? I've never exhausted my D750 buffer yet and shoot wildlife with it.
- D750 will require you to change DX lenses for FF ones - very expensive (I did this and can tell you it's a wallet unfriendly move)
- D500 has a pro button layout, while the D750 has the enthusiast one, similar to D7000, so easier to transition from your current camera
- D500 has the new Nikon bluetooth system (SnapBridge) to push jpgs from the camera to a smartphone/tablet easily; the D750 does not. Easier and faster to share but only for jpgs when likely you are shooting in RAW.
- D500 has a mix of SD (UHS-II) and QXD cards - not ideal; D750 has 2 x SD cards, albeit the slower UHS-I format; in any case you'll need new cards for the D500
- D500 has better video options including 4K support (albeit on only part of the sensor as it is heavily cropped)

My 2c.

For birds in flight I strongly recommend setting the camera to GROUP AUTO FOCUS (Gr P) in the top screen. Then pick the center focusing spot (this should be a default setting on your Nikon) and Nikon will automatically put four spots around that one for a total of 5. As long as, when you track the bird, one of the five spots is on the bird the bird will be in focus. Use center weighted metering, continuous auto focus, shoot in Aperture priority and at least two stops down from wide open and shoot at least 1/1600 second or higher. Good luck.

I recently picked up a used Nikon TC-20E lll teleconverter in like new condition to use with my 70-200 f2.8 lens. According to Nikon this converter is fully compatible with the 70-200 lens. Such was not the case. Without the converter the lens autofocus is fast and accurate. With the converter the autofocus is slow as molasses, hunts all over the place and cannot get a focus lock. I tried using it with both a full frame and a cropped sensor camera and with a few other f2.8 lenses I have, all with the same result. Needless to say(but I'm saying it anyway) I'm very disappointed.

Well I gave in and have been using Creative Cloud Photo for nearly 2 years, but kept my Corel PSP X8.
Well today I got a "survey" about Plan X and Plan Y.
Plan X Lightroom cross platform, on line storage and a "portfolio" on line feature for $9.99 a month.
Plan Y Lightroom Plan X + Photo Shop for $19.99 a month.

Looks like I may be dumping CC, using my standalone LR 6 and Corel PSP + Corel After Shot if they go through with this plan to change things during the next 12 months as mentioned in the survey. I really like LR and have been getting used to PS. But not $20 a month or $240 a year. I will concentrate on my Corel products and get used to them.

Them raising the price was one of the main things many of us were worried about. Well, these new "plans" are just that, a price doubling to keep using LR and PS.

And there I was just beginning to think that my normal pessimistic attitude was pleasantly wrong in this case!

We pessimists have an advantage, if things go well we get a nice surprise, if they go wrong we were expecting it and don't get crushed about it. Just pissed.