Im going to be getting either a new 300/2.8 or a used 400/2.8. Even with one new and the other used, the price significance is large. New 300 is about 38xx and a used 400 is about 6xxx. Im pretty sure I can get a good discount through Canon and their employee purchase program, but assuming I cant, which one do you think I should go for? With the 300.... I can use it for everything including basketball and it can be handheld if I need to. It can also be used for things other than sports. Throw on a 1.4x and its a 420/4 which is close to the 400. If I was shooting outdoors with the 400, Id probably be shooting at /4 anyways if there was enough light. Im sure that the 400@/4 would be sharper than the 300 with a 1.4x though. With the 400.... I can use it for just about everything except indoor sports. The pictures from it are just amazing. Ive never shot with a better lens. Put a 1.4 on it and now Ive got a 560/4! 2x and Im at an 800/5.6! Those are hard numbers to turn down in my book. However, its MUCH more money, Id probably be restricted to just outdoors sports photography or nature stuff (which I dont do..). In the end, I just feel like if I bought the 300, Id always think how great it would be to have a 400 and if I bought the 400 Id never wish I bought the 300 but I may have wished I spent less money So, if money was a factor, which would you go for?