Tracy B. wrote:
Ok, currently I have the 70D. My zoom is Sigma 18-300mm I also have the Canon 17-55mm (Love). I take mostly landscapes, and portraiture. The sports is only a couple times a year. I do macro with my 100mm also.
For "mostly landscape and portraiture" a 6D would be great! And, since you already have several fast primes (esp. for portraits), if it were me I'd fore-go an f2.8 zoom and just opt for an f4 or variable aperture "walkaround" zoom... either the 24-70/4L IS USM or the 24-105L IS USM or the newer 24-105 "STM" would likely be fine, but an even cheaper 28-135 IS USM might serve just fine, too. I use a 24-70/2.8L (first version) personally.... for portraiture, though, I actually like it better on a crop camera than on my full frame, where it always feels "a little short" to me.
I'd recommend doing whatever might leave some room in your budget for a wider landscape lens, such as the 16-35/4L IS USM or 17-40/4L USM or 20/2.8 USM . For landscapes, 24mm or 28mm just isn't wide enough on full frame for my tastes (I use a 20/2.8 presently... but am considering the 16-35/4, which is newer and sharper than the bigger, heavier and more expensive f2.8 version). To take full advantage of the FF camera, I would want to have a FF-capable wide angle lens for landscape, too.
Keep your 70D as a backup and for those occasional sports/action/wildlife shoots, where it's faster frame rate and more advanced AF system will serve well. The 17-55mm is a great lens on it, but longer is usually necessary for these types of shots and I'm not familiar with the Sigma 18-300mm... However I'm very suspicious of any 10X or 15X zoom, though it might do all you need for occasional use. Personally, I do a lot of this type of shooting and use a couple 70-200s (both an f4 and an f2.8) and a couple 300mm primes (also f4 and f2.8), sometimes with a high quality 1.4X (and, more rarely, a 2X on the f2.8 lens).
With the 100L, you're all set for macro on either FF or crop camera (personally I use the 100 USM non-L/IS version, along with several other macro lenses).
I would only recommend a further "step up" to the 5DIII if planning to only have it alone, selling off your 70D to help pay for it, for example. The 5DIII's faster frame rate and more sophisticated AF make it more versatile than the 6D.... BUT, if you give up the crop sensor camera and go FF only, you're going to need a much longer focal length lens for sports/action (and neither the 17-55 nor the 18-300mm will be usable on it). Where you use 200mm now, you'll need 300mm on the FF camera. Where you use 300mm now, you'll need 500mm on any FF model. A lot bigger, heavier and more expensive!