EricBoehm

I have a 40 STM pancake and also an old 630. If nobody gets back with a definitive answer soon I'll find a battery for the 630 and let you know.

I personally use the 24-105 and have no real experience with the 24-70. However, I think I can offer some insights or at least some things to think about. First, your 17-55 gives a full-frame equivalent angle of view that would be almost identical to the 24-70 (27-88ish), so you can get a rough idea if the focal length range of the 24-70 on a full-frame body would work for you. The tradeoff of one stop between the 2 lenses is more than overcome by the IS on the 24-105 in terms of camera shake issues. If that's the only concern, then certainly the additional reach of the 24-105 would be the obvious choice. I have read that the IQ on the 24-70 is better than the 24-105, especially once stopped down a little bit. Certainly, the f/2.8 of the shorter lens might work better for portraits where you're trying to blur the background a bit, but the tradeoff there is the shorter focal length. At 70mm, you're a tad bit shy of what many photogs consider the "ideal" portrait length. On your 650, both lenses would put you into perfect AOV ranges for portraits, so that issue is negated on that body, but so does your 17-55. Undoubtedly the IQ improvement of either lens over the 17-55 would be significant. As for dancing and group type pictures, flash will handle stop-motion with or without IS, since the flash burst is so short. However, there may be ghost issues due to camera shake that IS could help with. Remember, the shutter is open longer than the flash duration. So, the issues to consider, as compared to your current setup are this: Do you want to stay with very similar relative focal length (AOV) and f-stop values while upgrading to full-frame? Then the 24-70 is your answer. Do you want to get more reach and add IS, but lose a stop of light and arguably some image quality (as compared to the 24-70) in the process? Then go with the 24-105 IS. I'm personally quite happy with the images I produce with my 24-105, but I don't do the kind of photography you do. It's a very capable lens, but I have heard it said that it's definitely not the "very best" of the L series zooms. That having been said, it's not dissapointing in any way to me either. The IS is very helpful, and the images are tack sharp once stopped down a little bit. Distortion on either lens is easily managed in LR or DPP. I don't shoot much in the way of weddings or parties, although I was at a wedding just a few weeks back and got some very respectable images out of the 24-105. So there's that. :) I too shoot a mix of FF and crop-sensor bodies, currently rocking a 60D and a 6D. My go-to on the 60D is the venerable EF-S 17-85 IS, which aside from some pretty nasty CA (again, easily corrected) produces a very pleasant image. The go-to on the 6D is the 24-105 IS. Those are what I keep mounted on the bodies. I also have a whole range of primes, an EF-S 10-22, EF 17-40 L, EF 70-200 f/4 (non-IS), and a handful of others. If I were starting out, and for my kind of photography, I'd probably go with the extra reach and the IS and get the 24-105. As for primes, you basically can't go wrong with the 50/1.8. These days I might opt for the newer STM version. On FF, of course, it's "normal" AOV, and on the 650, you'd have a near-perfect portrait lens. Remember to consider that any of the FF lenses you buy you can also use on the 650, so be sure to consider those possible advantages too. Good luck!