For close to four years I've had and used a pair of 7Ds alongside a 5DII. The crop cameras serve well for sports/action, which is about 85-90% of my shooting. The 5DII is great for the landscapes, architecture, still life, portraits, macro... about 10-15% of my shooting. With the two formats available, I haven't had to jump on the 5DIII bandwagon yet, but probably will eventually. But even when I do, I'll keep using crop cameras alongside and they will still likely handle the bulk of my work.
If I could only have one, it would be the crop camera. Today's can make very presentable landscape & architecture shots up to 16x24" prints, but beyond that I'd prefer to work with the FF camera. There's more to it than just AF performance and frame rate. A crop camera also allows me to work with smaller, lighter lenses handheld... where to get the same "reach" with the FF, I'd have to break out the "big guns" and a tripod or monopod to put them on. Sure, I could do it if need be, but the croppers are working really well for me. I'll be interested to see what Canon comes up with next... between the 70D looking more like a 7D in many respects (AF system) and getting a whole new sensor. In fact, you might want to wait and see what the revues have to say about the 70D, instead of buying nearly 4 year old tech in the 7D. Of course, the 70D (and no doubt the 7D Mark II that will likely follow soon), will go for a premium price for a while and 7D or 60D will be much more affordable.
Yes, 5DII is better for high ISO/low light work. It's good for about one stop higher ISO than 7D. Plus the 5DII's AF system is still able to focus in a bit lower light, after 7D's has given up. I think Canon's rating of the two camera's AF for low light is a bit conservative for the FF camera and maybe just a little optimistic for the cropper. They rate both around -0.5 EV, but I find the 5DII is able to keep working a little lower light, though it does slow and hunt more.
Speaking of 60D, if considering the 7D, you also might want to look at 60D. Though it looks the same, it's AF system is better suited to action/sports than the 5DII's, better at tracking movement and all nine points are the "better" dual axis/cross type. Maybe by scaling back a little bit on the cropper you could afford to keep both formats. I'm sure there will be some blow out prices on 60D while they are clearing back stock, not to mention some deals on lightly used ones when people run out to upgrade.