Make sure you're happy to be buying outside of the big two (Canon/Nikon) before you go for the sony. I have a sony A560, which I upgraded from a Sony A200. I'm really happy with it but at the back of my head I know it's always going to be just a little bit more difficult to add to the system as Sony-fit lenses and accessories aren't as prevalent as Canikon. When I bought the A200 I didn't really have an understanding of the Canikon dynamic to the market and once I did, I did have second thoughts about upgrading.
If you ever aim to go pro I'd advise against Sony as they don't really have the same backing as Canikon.
All that said, as a hobbyist I love mine. Maybe it's just my perception but I think Sony make up for their smaller market share by packing in more features for the price. Image stabilisation built in to the body, and the speed mode giving 7 fps (albeit with some limitations) for example.
Last point - but have you actually handled both cameras yet? I looked at the A55 and just found it to be too small in my hands.