Unless one stop of shutter speed will make a huge difference for you, e.g. if you shoot indoor sporting events, I'd stick with the f/4L IS version. Even if you do shoot indoor sports a lot, I'm thinking your money might be better put towards something like the 135mm f/2L (which you can probably get for something close to your great price on the 70-200/2.8 - having a slower zoom and a faster prime in the same range is a good combination.