I'm having a hard time deciding between these two wide angle primes. I have a 7d and i'm going to acquire a 5d mark ii soon. I feel as if we can debate wider/slower versus tighter/faster. They're also about the same price. But my question is regarding optical quality. Would anyone know if one of these lenses is significantly better in terms of sharpness, CA, and the sort? If it's a tie, I think i'll pick the 20mm f/2.8