I use both lenses quite a lot. I can say that you will not be disappointed with either lens and you really aren't handicapped by your choice. I think the question of weight is quite relevant. I can handhold the 300 for an entire game if necessary, but the 400 is out of the question. Even on my monopod, the 400 is very heavy and very akward. I've compared the 400 to having a small child on top of your monopod. I'm not a small guy (6-2, 215) and I find the 400 hard to use for an entire tournament weekend. It's just that big and akward. The IQ from both lenses is spectacularly good.





The first picture is with the EF300mm f/2.8, the second picture is from the EF400mm f/2.8. Both ran front page...I think you can use either lens and shoot great stuff.