50D. Although Canon are trying to get the 500D away from the 'entry-level' brand name, in my opinion xxxD will always be entry-level as the xxD will always be designed at more semi-professional photographers. At the moment the 500D looks good, but when the next xxD (60D?) gets released it'll make the 500D entry-level again. That's my opinion anyway.
Basically, if you're planning on taking photography a bit more seriously go for the 50D. If you like the 400D in terms of size, would like to continue how you are and wouldn't mind video then get the 500D.
For me, it's 50D every time (hence owning one after the 500D was released).