100-400 IS L, easily. About the same price as both the lenses you suggest and has longer reach for motorsports. Unless you're right on the track (i.e. street circuit), get the 100-400.
If you are in London or thereabouts, why not try renting from Fixation - they have all the lenses mentioned above. Will cost you only about £30 for a weekend hire for each. I've used them - no hassle at all.
I too shoot a lot of motorsport with a 70-200 f4/l sometimes with my 1.4x TC. I have enjoyed great success with this method and got the lens instead of a 100-400. I am not disappointed with my choice. Generally speaking there is enough light at motorsport events, in the daytime, to take your pick of f stop and shutter speed so f4 limit is not a problem for me with my nice light lens. - Jono
Thanks for the comment, it really is a much better pic when viewed full size.
So, have you decided? I actually placed an order with B&H in New York for a 70-200 IS as a replacement for my F4. This would have benefitted my 'event' photography more than motorsport. As it happened they could not put more than $1600 on my UK credit card in one hit so i relented and got a 580EX flash and 1.4X TC instead. So, i still have my trusty 70-200 F4 and a great flash and a TC now. All these have benefitted me far more than the 70-200 IS would have i think.
Either way, its all a trade off, these different lenses would not exist if there wasn't a good reason to use them all at some stage! What lens to have..... You decide!!!
