Fact: both camera will produce excellent Image quality.
Never used the 550D before, but I dont doubt its quality at all. As for A550, the same sensor in A550 is being used in Nikon's Semi Pro DSLRs, it caused Pentax to ditched Samsung (its sensor maker) and use Sony Sensor, and LEICA also used the same sensor in their X1, it tells alot about the quality of it, dont you think?
Imo its all down to whether u need video or not. If u need, just get the 550D, no need to think. However if u dont need, then the A550 has much better functions than the 550D.
Pro of 550D over A550:
Video
Userbase: There are alot of Canon users, so if you have alot of friends who has Canon lenses and are willing to lend u their lens, this might be a consideration. But I hv to stress, the good lens usually dont get loan out even between friends. Unless u really are so their suck c*ck buddies ;p
Pros of A550 over 550D:
Build in Image Stabilizer. ALL ur lenses are stabilized. Image stabilised 30mm primes, 50mm primes, etc etc. U don get those with Canon.
Fastest Live View of any system (maybe except Olympus)
7 frame per second shooting speed (550D is only 3.7fps)
Auto High Dynamic Range (no tripod needed)
Tiltable LCD screen for odd angle picture composition
Build in Wireless flash support
there are alot of mention on Sony having "limited lenses". This statement is true, yet its totally wrong as well. Sony does hv limited lenses, but thats because alot of Sony Alpha lenses are branded under Minolta. Minolta is Sony. Minolta lenses = Sony Alpha lenses. And there are hundreds of Minolta lenses available to choose from, and they're usually very cheap as well. So lenses selection will never be an issue no matter whether u choose Canon or Sony.