T2i/550D, T3i/600D, 60D and 7D all share essentially the same 18MP sensor.... not exactly the same, but close enough you'd have a hard time telling apart images from each of them. Same resolution and high ISO performance. It's more the other features of the camera (price, size, build, control layout, feature set, performance other than image quality) that set them apart from one another.
In fact, I always felt my 15MP 50Ds gave very close to 5DII image qualities: color rendition, etc. The FF camera has a bit more dynamic range than any of the croppers, and FF will always give more fine detail to work with, and stays more noise free at about a stop higher ISO. But to my eye the later, 18MP croppers just seem to saturate colors a bit more (more "Nikon-like", for lack of a better term). Of course, you can tweak this either with the camera settings or in post production, making the results more similar with each other.
The T1i should be pretty close to the 50D, too, since they share sensor and processor.
All of these cameras use Digic 4, though they probably each have their own unique algorythms.
The metering system of 50D, T1i and 5DII is the earlier 35 zone type. The other models all use the newer, 63 zone iFCL type metering system, which might make a bit of difference in some specific situations (I find it handles strongly backlit situations better in my 7Ds).
As good as the last couple generations of crop cameras have been, I'm not sure a lot of folks will ever really "need" full frame. The differences just aren't that dramatic any longer, unless you are doing huge prints or massive crops or have other specific need that only FF can fulfill. I prefer FF for subjects that I'm more inclined to print really large, or when I want to work with really large or really small apertures... But still use croppers for the vast majority of my shooting and have sold prints as big as 24x36" from images made with both the 15MP and 18MP APS-C cameras.