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.
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?
3. I did like using the 5DII for the short time I had it and the images were very nice, especially since I almost exclusively use the center AF point, but I do use the high-speed mode a lot on my 7D and I will certainly miss it (and again, cannot afford the 5DIII).
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.
I think you've already touched on the fact that the benefit you will get will depend on the use cases you need to fulfil.
Having access to a 7D, 5DII and 5DIII, I tend to gravitate to the 5DIII or 7D for anything that moves, and I'm happy with the 5DII for subjects like landscapes or posed portraits.
In low light, the 5DIII's AF will lock in circumstances where the 7D does not stand a chance.
If you can afford to keep your 7D, the 5DII +24-105 f/4L IS USM kit may be a good option to start with. I still believe you cannot go wrong with the 24-105 f/4L IS USM. I use that lens a lot on the 7D now as well.
If indoor photography without flash is your main consideration for full frame, you could even start with a 5DII and 50mm f/1.4. (Before the days in which we all got lazy with zoom lenses, very many people used to shoot mainly with a 50mm lens.) I realise this approach would see you have two separate cameras, but it may accommodate what you need. You can then slowly start to acquire the glass you want for a full frame body.
If a major use for you is landscapes, then there is often no real differentiator between the 5DII or 5DIII - especially if you plan to shoot at ISO100 and use manual focus.
In terms of low light performance, the 5DIII really starts to pull away from the 5DII above ISO800. - My wife has a theatre job lined up later this month and the 5DIII will be a real winner here, as the 5DII struggles a little at ISO6400, while the 5DIII's ISO12800 is very usable.
The advantage you have at the moment is that Canon has played its hand, as far as full frame announcements go, with the exception of a possible very high resolution very high price body. You thus know what Canon will have on the market for the next 3 to 4 years. You know that the 5DII will be available until roughly the end of the year, so if you want one, you have a limited period of time left to buy.
As far as low light work goes, I tend to usually reach for a flash if I can, to augment whatever ambient light is available, unless flash is not permitted. What the low light performance of the new bodies does give you though is a lot more flexibility to blend ambient light and flash.
If you do opt for the 5DII or 6D, I suspect you may still want to keep the 7D, because of its build quality (it can take longer showers than the 5DII), AF system, 8fps and 1/250s maximum flash sync speed (useful for outdoor flash in bright sunlight when you want to overpower the sun).
What you lose with the 5DII is full integration with some of Canon's latest accessories, like the GP-E2 and some of the capabilities of the new 600EX-RT Speedlite and ST-E3-RT transmitter.