I own both, and while both are good cameras, to me the differences come down to whether you want to use a camera that is keeping with the form and functionality of Canon's 'professional' series of cameras, or you don't care about that.

Canon 50D

• Feels more solid (more metal vs. plastic)
• Screen on top
• 2 adjustment wheels (550D only has 1)
• PC connector for a flash
• AF points are generally more responsive
• Brighter viewfinder (uses a pentaprisim instead of a pentamirror like the 550d)
• Higher burst rate (6.3fps vs. 3.65fps)

Canon 550D

• Wider LCD screen
• More megapixels (18 vs. 15.1)
• More advanced metering system (iFCL)
• 1080p video

Generally speaking the advice that I'd give is this: if you need to shoot video, or you really need the 3 megapixel jump up to 18mp, then go with the 550D... Otherwise, all things being equal, the fit, finish features and layout of the 50D are more in keeping with what is available on the higher-end Canon bodies As a 'non-casual' photographer over time you're more likely to migrate up to a body higher in the Canon food-chain, instead of down to a more consumer-oriented one, and with the 50D you'll already be used to the layout and operation of all the knobs and features of the 'pro' line of cameras (for the most part).