If you really want the faster lens, I would recommend it, take a look at the Sigma 70-200mm f/2.8 EX DG HSM lens. Excellent lens. I tried both the 70-200mm f/2.8's from Sigma & Canon(non-IS) and the Canon 70-200mm f/4 and decided the f/4 wasn't gonna cut it but the f/2.8 Canon was too much, but the Sigma fit in the middle and performed just as well as the Canon.
I went through the same decision before I bought the sigma. I got a deal at a local shop, and was able to fully test it before buying.
There have been numerous times where even outdoors, the f/2.8 has been a huge success on a darker cloudy day. I don't like to bump the iso up too much, I really don't like noise at higher iso. For action shots, you can get away with bumping the iso up there at f/4, but the f/2.8 has been a huge success!

You were the one holding the Sigma lens in the Popular Photography Magazine Ad weren't you?

