FEChariot wrote in post #16699391
However, what do you need it for? If you are shooting with lights in a studio at 5.6 or smaller, then there isn't much reason to carry the extra weight and spend the money on it when the f4 version is fully capable. Likewise the 135/2 gives an extra stop so it might work for you too.
If I knew it was going to stay in a studio with lights there'd be no reason to not get the f/4 non IS in my opinion. I don't forsee myself shooting indoor sports much, but having the 2.8 is nice to have when I need it. I used the 2.8 II at an indoor cheer competition this past weekend. I didn't need the IS during the competition since I kept the shutter speed at 1/400 and shot wide open, but I did use it before the teams went on to the floor during warm ups "backstage" to get some quick portraits of them. Did I need to take those portraits at 200mm and 1/125? No. But I did...because I could. 
I won't lie, do I need the 2.8 for the bulk of what I shoot? No, but I want the 2.8 over the 4. It's just a matter of which 2.8.
Here's a sample shot from the day using the rented 2.8 II. I'd have never been able to get this shot (or any of them during the course of the day) with the f/4.