The 5D is going to be the clear winner here for everything other than the screen and sheer resolution (but see below).
The fact that it's full frame means you get more depth of field flexibility -- you can go shallower than you can with a crop camera.
The autofocus system, while older, is still better than what's in the current generation Rebels except for its layout. The 5D center point has hidden expansion points around it that help it for low light as well as for servo tracking. The downside is its layout -- the focus points are clustered around the center, so you have no "rule of thirds" coverage with it. You will be doing the "focus-recompose" dance more than with the Rebel.
Based on what's been reported around here, the image quality will be roughly equivalent at high ISOs, and better at low ISOs. In fact, some regard the 5D's low ISO image quality as nearly the best that's out there -- even better than that of the 5D2. The antialiasing filter is very weak, so images look very sharp and detailed. That will partially offset the real resolution difference. The larger size of the sensor, combined with its lower resolution, means the 5D isn't going to be nearly as demanding of glass resolving power as the 600D. The 600D's ultra-high-resolution crop sensor is extremely demanding of glass, and only the best is capable of resolving well enough to match the capabilities of its sensor.
The 5D has already seen the bulk of its depreciation, and while it will probably continue to lose value, it will do so at a substantially lower rate than the 600D would.
No, in this case, if it were me, I'd be going with the 5D for sure. There may be an apparent difference in the price of glass, but it is apparent only. There does exist very inexpensive glass for the 5D that is nonetheless still good, such as the 28-105 f/3.5-4.5, or the 28-135 f/3.5-5.6 IS.