MicrostockGroup

Why not rent them to try out for a week before you decide?  www.lensprotogo.com I have used in the past and he has both those lenses in stock available to rent.

Mat

If you're going hand-held, just get the faster one! Also depending on your climate, eg. I'm in Ireland, it's ALWAYS low-light, so I have to go with faster lenses.. 300mm is plenty long anyway!

Hi Xalanx,

 I think Mat makes a good point about renting for the day. I used to shoot sports when I first started out and I think the more the better I would go with the 400 but don't they offer the 400 in an IS model. That would be the only bummer. Shooting at lens focal length would have you shooting at 1/400th which can be  tough on a grey day especially at 5.6.
 I have a 300 2.8 if you would like to see a shot from it I would be happy to send one along if you want to PM me. I will say when shooting in tight with my 300 there is no way to shoot wide open and carry much depth so I wouldn't think shooting at 5.6 with that length would be much of a loss except for the extra stop for shutter speed. Tough choice but again I would rent them both and put them through the grinder first. Good luck and have fun.

Best,
Jonathan