Bro, maybe I can give u some help..

Got my 500D last August 09 never fail me till now, do 90% indoor portrait with control lighting IQ is good. Just got my 550D on Wednesday love the HD movies function, better IQ under same lighting set up, same len Tamron 17-50mm F2.8 or Canon 70-200 F2.8 ( more detail on model face detail) Otherwise, both are great camera. I love the 500D so much, so keep the 500D for my elder son so he can pass his 400D to the younger brother...haha
If u like HD movies shoot (with better control), pay a bit more get 550D, pure photo shoot 500d will meet your need as compare to 550D.