so i'm looking around for a "bang for the buck" lens and i've arrived at these three choices. now the question is...which one? i already have the tamron 18-270 and i plan on getting a tokina 11-16 f/2.8, and maybe a tamron 70-200 f/2.8 with some extension tubes. tamron 17-50 f/2.8, or tokina 16-50 f/2.8, or tamron 28-70 f/2.8? any sigma lenses i should consider?