For months I was completely torn between the 50D and the 550D. Time after time I would go into the camera shop and compare the two, but kept coming to the conclusion that both had some of the functions I required, but neither had all of them. So in the end I stumped up and went with the 7D.
Things that were important for me that the 7d had, but the 550D didn't were; faster/longer burst rate (I was beginning to shoot more sport), custom modes (from previous cameras I had gotten used to have a couple modes set up for different functions), no rear menu wheel or joystick for fast changes (i.e. shutter speed/aperture size in manual or changing focusing point), and a real biggy for me, the 7d (and 50d) just felt so much better in the hand than the 550d. I know that the modern plastics in the 550d are probably perfectly adequate for the job, but it just felt too light and plastic'y to me when compared to the other two.
With sensors/metering/focusing systems being so good these days, the main difference between cameras (excluding sensor size), is the amount of options a camera will give you and the build quality. In the main most of these options wont improve image quality, but will instead make life easier for you in certain situations, but it does mean that to get the best out of these options you will have to spend to learning and testing them. So it comes down to a trade off between ease of use and flexibility.
Obviously not every thing (or anything) I listed will be important to you, and if the 550d does everything that "you" require, then you will have a really decent camera at a fraction of the price of some other cameras.