As already noted it really will come down to how you anticipate using the lens. I think all Canon shooters should at some point seriously consider getting one of Canons four 70-200s. The question then becomes which one. I plan on adding one of them hopefully within the year and currently Im leaning towards the 70-200 f2.8 non is.
As Eric noted I think the primary consideration is wether or not you anticipate using for stationary or action shots. Second to that Id add indoors/outdoors and finally whether or not theres any advantage/disadvantage to the