18mp sensor of 550D performs better than 15mp sensor of 50D in terms of metering and high iso noise. so, sensor wise, 550D is a better body. if you more into landscape/macro/portrait, then you have already got a good body. you need good fast lenses to extract maximum capability out of that sensor now.
if you are more into sports/action/wildlife, then you need a faster focusing system with better frame rate. here a 50D or a 7D might help. but then also, you would require top notch glasses to get benefit of the body's performance.
so, i recommendation would be to understand your need according to your shooting genre and then only upgrade body or buy good glasses.