I've been pretty happy with my 50D kit for general travel/nature/event photography, but I've been wanting to pick up a FF camera to achieve shallower DOF more easily for portraits and lower noise when shooting indoor events (e.g. my son's plays in dark theaters). I downloaded RAW samples from DPR for the 50D, 6D and 5D Mark III and played with them all in LR 4, and I was shocked at the difference between either FF and 50D (easily 2 stops in noise and better contrast across the board). I was leaning towards a 5D mark III, but I just can't justify the extra cash (as an amateur) and don't want the extra bulk and weight for travel, so I've settled on a 6D for its smaller size and GPS.
My dilemma, though, is whether to buy the kit with 24-105L or skip it and put the $500 towards a Sigma 35 1.4 HSM. I already have a 17-40L, 70-200 F4is and 85 1.8, so that wouldn't leave too big a gap in FL, and since what I'm really missing with my 50D is DOF control the large aperture 35 seems really appealing (particularly after the reviews I've read here). The 35 at 1.4-2 looks better than 24-105 at 5.6-8 to my eye. I used to shoot fixed FL lenses exclusively in my film days, so I know the pluses/minuses of that from a creativity standpoint and have no problem "zooming with my feet".
Any opinions of 24-105 vs 35 1.4 for a walk-around would be appreciated. Either way I intend to keep the 50D for now (probably bolted to the 100-400L for zoo/wildlife work).
For a walkaround the 24-105 is brilliant. To get creative, or low light, or street,- get the 35/1.4. I have both and they are both great, but with different uses. I had however a strange flarephenomenon that affected sharpness on the 35, as I wrote about in another thread here. The result is that I don't use it in studio, but for many other situations.