I have an APS-C setup with the 7D and 10-22 and 17-55 2.8. I have considered moving to FF with the recent reduction in prices of the 5DII and the 6D being announced, but here's the dilemma:
1. I am quite happy with the IQ of the 7D up to ISO 1600, but the noise above that is bad. So indoor photography without flash suffers. That is the main reason for my FF considerations.
Some people will disagree with me but this is from my experience, you only gain 1-stop of light in terms of usable ISO with the 5DII. Is that enough for your indoor photography without flash? Additionally, and I think this is very important, unlike the 7D the 5DII does not have the focus assist beam. You will need to attach the IR flash trigger or any speedlight with built-it IR focus assist beam. This is important for lowlight even photography. SO factor that into the cost of owning a full frame Canon.
2. I am aware that if I move to FF, I will have to trade my EF-S lenses for FF equivalents. My lenses will hold their value and I can get the 24-105 and 17-40 without losing any money (cannot afford the 24-70 II). But while the 24-105 is very good lens, I am not so sure about the 17-40. I didn't like the copy I owned (and I used it on a 5DII as well). And, I cannot afford the 16-35 II. Additionally, if I use f/4 after moving to FF- what do I gain over using f/2.8 in APS-C?You own both of the best lenses for crop-sensor Canon bodies. I just recently upgraded to full frame with the 5DIII. I'm having a hard time replacing the EF-S 17-55 and the EF-S 10-22. I haven't yet found zoom lenses that are just as sharp as those two lenses. To date, I have tested 17-40 (hated it), 3 copies of 24-70L mk1 (the 3rd one was okay but still not sharp on the edges like the 17-55 and I sold it last night), and the 24-105L (worse than the 24-70 on the long end 50-105mm). I just ordered the 16-35II and I'm saving up for the new 24-70II or the just announced 24-70 F4. I do photography as a hobby and I'm just an enthusiast but I print my pictures (only sold a couple of prints).
My point to this is, unless you're printing your photos (landscape, portraiture, fine arts) I don't think there's much benefit for you to upgrade to full frame. You will be very disappointed with the 5DII coming from the 7D. It also sounds like money would be a factor in your decision. If you're unable or not willing to spend more money on the more expensive Canon's new lens offerings or prime L lenses, I suggest keep the 7D and the two fantastic lenses you already owned. If you do decide that you need full frame (btw, the bokeh is better in full frame), I suggest either save up for the 5DIII or switch to Nikon D600. Since you're going to have to sell and purchase everything, that's like starting from scratch, you might as well switch to Nikon.
So given these circumstances, is there something I would gain by going FF with a 5DII/6D with a 24-105/17-40 (that offsets my loss of fps) or should I wait until I can afford the 5DIII/24-70II/16-35II? I am sure many of you have gone this route- your advice will be greatly appreciated.
You gain image quality (lower noise, richer colors and contrast, better bokeh), larger and brighter viewfinder and more complicated AF system. You will also gain larger file sizes. You didn't ask about what you will lose but here it is - you will lose reach from your tele-lenses, lose the IR focus assist beam, lose the ability to trigger off camera flashes, need better lenses therefore you will lose money. Lose more money to replace the gadgets that you have on the 7D but will not have on the 5DII or 5DIII.