When I was gettin' my 500D I could have picked a 50D instead. I tried both cameras and decided on a 500D instead. I don't know why it's such a big deal for all the folks to have a "metal" body. Yeah it's better built because it's metal but who drops their camera around so that it breaks?
For me the biggest factor was camera mobility, and for that I picked 500D. It's smaller than the 50D due to the absence of the top LCD (which is useless for me, thanks to the viewfinder/rear LCD). 50D is a LOT heavier and bulkier than 500D which makes it bad for carrying around your neck for 5-6 hours a day. A friend of mine picked up 50D instead, and now she regrets it because it's too heavy.
And besides slightly higher fps, other spec considerations are too minor between the two.
60D is interesting for now, but Canon keeps coming out with more "rebels" every year. We'll have to wait and see what they will come out with next.