A57 vs A580

Hi. Once again looking for advice. I recently went full frame and purchased Canon 5D MK3 with L lenses. Superb shooting machine but (there is always a but, isn't there?) Full frame is very heavy for my aging body. I was in my local camera shop where another prospective customer was trying out an A57 with 28-75 f/2.8 lens. he eventually left but I was impressed with the construction quality and after playing around with the combo was very impressed by the speed and accuracy of focus. The combo is a great size and weight for me. I have an old A580 with 16-80 Zeiss lens which is a nice combo but have become spoiled by the newer sensor capability. Anyone who has shot with both and can give me your viewpoint on this subject? I should mention that I shoot aperture priority, never use flash, take many indoor shots of cathedrals at fairly high ISO's, etc.
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LenG