How needed is the Image Stabilization on the Canon 70-200 f2.8 L USM lens?

I just picked up photography a few months ago as a hobby (not planning to go professional at all), but will be getting good gear and glass over time. Right now I have a T3i, which I'm considering upgrading to a 7D or probably a used 5d Mk II next year.

For my next lens purchase, I'm considering a 70-200 L lens, I'm just not sure which one to go for.

I'm deciding between the 70-200 2.8 L with no IS, or the 70-200 f4 L with IS.

My thinking is the following:

1. 
   

   While I do not plan to photograph weddings or anything like that, I do plan to use the lens to take pictures of my kid on school plays over the years, so I'm guessing the 2.8 would be useful there? He's not on school yet, but I hear flash is not allowed there? How would the f4 fare in such situations?

   
   


2. 
   

   I have the 18-135 kit lens with IS...I turned off the Image Stabilization on the lens, and was able to take decent pictures handheld with not-so-fast shutter speeds (1/15, 1/20's), although I do NOT have a good pulse. Would being able to do this translate well to using the 2.8 with no IS?

   
   
   


3. 
   

   Most of the shooting with the 70-200 will probably be done outside..just that case of shooting plays or a concert or too.

   
   

So..what should I do? Anything I'm not considering?

Answer

according to your needs the f/4 should suit just fine, but since they are basically the same price I would go for the f/2.8... i have it and its an awesome lens.

take a look at this

http://www.the-digital-picture.com/Reviews/Canon-EF-70-200mm-f-4.0-L-IS-USM-Lens-Review.aspx