Just to let you know, that with the 5D3 I noticed that some lenses that are very sharp on the 60D were softer on the edges. But you really need to pixel peep. The lenses that I noticed this are 70-300L and the 24-70L
Interesting. I really like the 70-300L on the 60D - it can be soft at times (especially near the 70mm end) but it really shines at the critical 300mm f5.6 setting. Is the key difference just softer edges?
I don't think you'll need the 24-70 if you already have the 24-105. The 24-105 would be a great walk-around lens for the 5D3.
I already use the 24-105 as my walkaround on the 60D, but it is certainly unique in that sometimes it is not wide enough to get a whole scene in, yet it is also not quite long enough to isolate a subject in the near distance. But it is the best compromise lens IMO, and the variable apertures on the EF-S options (e.g. 15-85) killed them for me. I take it the 24-105 would give a much wider perspective on FF and thus would make it a somewhat more versatile lens.
Actually, the reason I would want the 24-70 is for a replacement to my 17-55. I don't think I would use the 60D for wide shooting any more if I went FF, so that's why I would likely look to give it up. The f2.8 vs. f4 would likely make a difference for indoor social events, just like it did when I used the 70-200s. The 60D (if I choose to keep it) would exist solely for use with the 70-200 and 70-300 in order to get maximum reach out of those lenses.
Another lens in question (just thought of this) would be the 35 1.4. I LOVE this perspective on the 60D, very natural FOV. However, on a FF this would effectively become more of a wide angle. There's the question of whether that would suit my style (also considering that I might also later have the M with the 22mm lens which is the same perspective), or whether to trade it in for a 50 1.2 or even an 85. Could even help fund the new 24-70 along with the 17-55...
With regards to the 6D, I agree with you that it doesn't seem much of an upgrade. I think I would rather get the 7D or the 5D2 to get more value for the money.
It's unfortunate...I didn't like the way everyone was trashing the 6D without so much as a preview, but that said, I was hoping for a little more based on the price point. If they priced it at $1500 (identical specs) it would be a much more attractive option. At $2100 it's almost tempting to look for deals on the 5D3...