I know you have your choices but why not go for a used 400D or 450D or even a 40D?
I don't know what the prices are over there but I am sure you could possibly get a used one for a bit cheaper and get a better lens instead of the 18-55. Just my opinion though. You could also get a used 40D, like the one I have, for maybe the same price as the 500D or 550D instead of going all the way for a 50D as the 40D is not much different from the 50D. Sure it's a little different in terms of a few features and the MP but if you aren't bothered about that then it's a good option. Up to you though, I always think glass before body. So basically as I already have a 40D, I will not be upgrading anytime soon on the body but will instead upgrade the lens. Hope this was of some help for you. ;)
P.S - Oh and I agree with what Mark said about the 40D, it's probably better than both the 500D or 550D apart from the video recording thing, it's a great all round camera and many people still buy them over the 50D.