Which would have the better image quality - a Sigma 500mm f4.5 vs a 300mm f2.8 AFS I with TCE17? The primary subject matter would be birds. I am looking at a used sigma and a used 300/2.8 afs I. I already have the 300 f4 BTW but need more reach. I rented the 500mm f4 vr for the launch and that has given me the desire to get more reach. I found a number of threads on the sig 500/4.5 over at POTN. The lens wascompared to canon 500 and 300 with TC but i dont see much comparing it to the Nikons.