Personally I would opt for the A580 over the A55 since I prefer the optical viewfinder, larger body, longer battery life, and have reservations about SLT tech. The A55 does have some clear advantages (video AF, higher FPS, no slap, etc) thanks to SLT and some prefer the size and EVF. I've never used either though so others might offer better advice.
Since you're switching systems it's worth considering Canon and Nikon as well if you haven't already. Both have their advantages depending on your needs, budget, and preferences.