I had a similiar descision a few years ago and went for the 70-200 2.8 non-IS over the IS. I was looking for multipurpose daylight and astronomy lens.
Some of the research I did included looking at MTF graphs supplied by Canon and also actual comparisions done by photozone, which show the non-IS lens to be sharper lens. One other thing that put me off the IS version was Jerry Lodriguss's test which showed it to be quite poor at f2.8
http://www.astropix.com/HTML/I_ASTRO...TS/C70_200.HTM
Having subsequently got the non-IS 2.8 lens I can definitely say it performs better than the lens Jerry Lodriguss tested in the link above. Thats not to say the non-IS is perfect, but it does a very good job.
Terry