Ok... One more comments to the Canon 70-200 f2.8 vs f4...
The f4 is very light compare to f2.8. As a result, it might also help you better handheld with less handshake. So the f4 really have advantage into slightly lower shutter speed to compensate the smaller aperture... And not surprice that if you can handheld better one stop with similar shutter compare to f2.8.
And if you only has 350D/400D/450D, I would suggest f4 version because of weight balancing issue.
Some might ask, how about 300D users? I would say sell off that camera because of lousy autofocusing sensors and upgrade. It might degrade the reputation on Canon excellent 70-200 series.
The only thing I do not like the f4 version is the lack of tripod collar... where you 'might' be needing for nite scene, long exposure kind of shots. But then again, as long as you buy solid tripod, should be ok screwing the camera to tripod with this lense. I think 24-70 L might be heavier and people also screw camera on tripod... Also on wat...
Any of this 4 lenses do rock... cannot be mistake...
And try to avoid the old magic pileline 80-200 f2.8 L. Optically fantastic, I used to have 2 copies, and manage to drop and kong 2 copies

The problem with this old lense is that no part are servicable... And I managed to confirm that with complete losses

It mean if anything kong, you eat banana, just like me...
Hope my sharing help abit here and there...