Which is better f4.0 70-200 IS USM or f 2.8 70-200 USM or the same cost?

[Skirball]

If you need 2.8 then the 2.8 70-200 USM is undoubtably better.

All five of the Canon 70-200 are great lenses, with their own benefits and "specialties".  Both of these lenses are quite sharp (all of the 70-200 are sharp).  Many people prefer the 2.8 versions, because they want that speed, though most probably go for one the 2.8 with IS. That's a slippery slope, because the 2.8 IS (Mark I) is known to not be quite as sharp as the f/4, so then they start looking at the 2.8 Mark II, which is twice the cost...  and one of the best zoom teles on the market.

Others prefer the reduced size and weight of the f/4 with IS, which is known for superb image quality.  The choice lies in your needs.  If you're not going to be using it wide open, then go for the f/4.

[RobertTheFat]

My rule of thumb is that an f/2.8 lens is an indoor lens and an f/4 is an outdoor lens. I got a 70-200 f/2.8L IS2 for indoor event photography, because I can cope with a lens that heavy when I'm mostly standing around. I've used it outdoors, notably on a visit to Acadia National Park a couple of years ago, but it's a handfull when you're climbing over rocks and trails. Outdoors I rarely need f/2.8, and a lighter, smaller f/4 (at any focal length) is welcome. Accordingly, I'm considering the new f/4 version of the 16-35 because I rarely need a WA lens indoors. (If and when I do, I can use my Tokina 11-16 on a 7D.)