If you're just looking for a family video camera or something similar, then go with the Sony A33 - continuous proper AF during video makes it infinitely easier, especially for non-keen videographers (vital for a family/memory video camera rather than full-blown cinematography).
If you're happy with manual focus, then the D3100 is a perfectly capable camera; the next step up would probably be a 550d, which aren't quite trickling down to the same price range just yet. Just bear in mind the Sony SLT's have an EVF for video usage, in body image stabilisation and much, much better AF during video, if any of that matters to you.