On paper there does not seem to be a huge difference between the two, however the 50D is a much better build so is more robust, it has a faster burst. However the 500D is lighter and does have a video shooting capability if that is important. The two cameras are essentialy for two different markets.
I guess you need to make sure that whatever you go for ticks all the boxes for you and addresses the issues you have with the Sony.
Here is a comparison.
Rewinding a little what don't you like about your Sony as it is possible that fellow sony users on the forum could help you to resolve them.
If you are going to change then have you considered Nikons at all? I am a Canon man by the way, 350D, 40D and more recently 7D. I am just conscious that if you do decide to change now then you need to ensure you make the right choice.