It kind of depends on the situation. Say you are shooting people acting on a stage and have to use 200mm. Assuming the 1/fl * crop thumbrule, you need 1/320" to shoot with the 2.8 non IS to get no shake blur. However, you could shoot at 1/80" because the people are not moving around that fast. In this situation, the f4 IS would give you 1 stop advantage.
Now say you are shooting a basketball game indoors and need 1/500' to shoot the action, Here the 2.8 non Is is the clear winner.
My solution here is to go with the lightweight on camera option. I have the f4 IS and when I need a larger aperture, I go to the 85/1.8 and in the future, I will add a 135/2 for more reach.