I was in a similar position, and went for the 70-300. I do a fair bit of wildlife, and for shy animals even 300mm isn't really long enough a lot of the time! The IS is a massive plus, I didn't think it was too important till I switched it off, it's VERY noticable! I plan to get a 70-200 f/2.8 in the future, but I'll still be keeping the 70-300 if I can. For me, the f/4 wasn't enough to counter the loss of 100mm and IS.
You could always get a secondhand one from here, they hold their value well, then re-sell if you decide to go for the 70-200?
Chris