Stephen,
If it's down to the 50D or a 550D, then, your choice will primarily be influenced by the ergonomics of the body.
The 50D is in-between the xxxD series of bodies and the 7-/5D series of bodies.
The 550D is an 18 megapixel system compared to a 15.1 megapixel system of the 50D. Much of a muchness, really.
Both the same sensor size, same ISO capabilities.
50D will have better focusing abilities owing to its cross-type sensor focus point, it also has one stop faster shutter speed (1/8000th).
The 550D has greater exposure latitude with +/- 5 EV compared to +/- 2 EV.
If you're going to be using the camera for sports or for capturing action, then, the 50D is better suited at 6.3 FPS, compared to 3.7 FPS.
If you're interested in making video, then the 550D has video capability.
As mentioned earlier, the 550D has a gorgeous preview screen. But, then, is this really all that important?
Lastly, the 550D is about 30% lighter in terms of weight.
My money would be on the 50D due to its feel in my hands. The print quality would be very, very similar out of both systems.
Bugger of a decision, Stephen. Good luck.
H