no offense guys but if you did a search that would be really good for you... BUT since you did post this thread I will give you my opinion on the matter... IF you want a 2.8 get it. If you dont have the cash nor want to spend the cash on the canon 70-200 2.8 IS, dont spend the money on the 2.8 non IS. Get the simga 70-200 2.8. It is just as fast focusing and as sharp as the canon 2.8 IS. The only thing you dont get is the IS.
As far as the f4 vs f2.8 is concerned, just remember that you dont have have the chance to go 2.8 at all on the f4 so that kills that. If you get the IS f4 it still doesnt matter because the IS will NOT help you stop motion. It simply allows you to use a slower shutter speed to get the amount of light in you need. If you shoot sports, indoors especially, you will want the 2.8. It allows you to use a faster shutter speed to stop motion.
Honestly I dont recommend anything I havent used before because that means that I have no reason to base an opinion off of it. But I have used/owned every 70-200 out there, including the sigma and tamron. Here is the chart that I have come up with as far as the order you should be considering these lenses:
1) canon 70-200 2.8 IS
+Best out there- has IS and a 2.8 aperture, great build, sharp
-heavy, but not as heavy as everyone makes it out to be but it is 3.2lbs, price
2) Sigma 70-200 2.8 EX DG APO Macro HSM II
+Fast aperture, focusing, great build quality, SHARP, price
-can sometimes have focusing issues ( I never had any and alot of people have never had any)
3) canon 70-200 f4 IS
+New age 4stop IS, Good build, Sharp, L quality (for those of you who care about that)
-constant f4 is not fast enough for alot of indoor sports or night shots.
4)Canon 70-200 2.8 non IS
+Great build, sharp, 2.8
-Doesnt have IS, heavy but not as heavy as the IS version
5) tamron 70-200 2.8
+Sharpest of the bunch, constant 2.8, price
- Uses a micro motor and isnt fast focusing, build quality isnt amazing
6) canon 70-200 f4 non IS
+good build
- no IS, constant F4 (for the reasons stated above)
Hope that helps.
David W.