Nikon d3100 vs d5100 vs d7000: Nikon DX SLR (D40-D90, D3000-D7100 ... Guys........asking for ur opinions.... which one will be better? nikon d3100 with tamron 17 50mm f2,8. or. nikon d5100+18 55vr nikon d7000+18 55vr ?