My suggestion would be to go to the store and try the two lenses (f2.8 non-IS and f4 IS) on your camera before deciding. I started out with the f2.8 non-IS version (Sigma), but ended up selling it and buying the f4 IS instead. There were a couple of reasons for this. First was the weight; there's a pretty big difference between the two. I found the f2.8 very heavy to lug around for any length of time, and often left it at home just so I wouldn't have to carry it. This may not be a factor for you, but it was for me. My second problem was the lack of IS. Much of what I shoot is moving, but for the rest, my keeper rate for non-moving subjects in less than stellar light wasn't that good with the f2.8. When I got my f4 IS, the keeper rate went way up. Don't underestimate the importance of IS in a lens in this focal range.
Lastly, when I bought the f2.8, I'd thought I might use it for indoor sports, but in the end even f2.8 wasn't enough in some venues. You might want to check that out as well before choosing.
I've never regretted my decision to get rid of the f2.8 and buy the f4 IS. I get more keepers, the lens gets used ten times more frequently, and the results are just stunning. My favourite lens by far.