If you're able to use the focal length the 135mm f2 is an awesome lens.
If you're not sure, try renting from lens rentals, you can rent the lenses and bodies that you want to try out and even purchase them if you really fall in love with them. They really have a good business model.
Also, if you're shooting sports for your dogs kids or whatever...you'll eventually get a 70-200mm f2.8 - resistance is futile - canon has an older model that I have and a new model with is.
Sigma makes some fine lenses too, so don't forget about them. My friend uses a sigma 70-200 f2.8 that he used for his kids high school football games on a 7d and he gets phenomenal images shooting av.
If you google fred miranda there's a lot of reviews of lenses on his site, but it's best to try one in your hand via rental or borrow a friends than rely on total webary.
