As someone who's used both cameras I have to say I prefer the handling on the EOS 550D, but that's personal preference. Like the other guys have said, try handling one before you make a solid, final decision. The EOS 550D takes some excellent photos but you wouldn't go far wrong with either.
Do you have a particular type of photography in mind?
I would be mostly using it for everyday photography, mostly static cars. I be using it mostly using it it in sunny weather. This will be my first DSLR but would also like the ability to video if I need to. I have decided to go for the Canon EOS 550D. This is partly because it gets better reviews than the Sony. I will also eventually become a very keen photographer, perhaps even full time so I feel by buying into a Canon system I would be better suited for future intentions.
Its a good choice. The Sony is much better for high speed work (7fps shooting) but the Canon is higher resolution, has great video (the Sony doesn't do video) and as you say, a a much bigger system of lenses and accessories.