[RichK]

First off, I shoot with Nikon gear, but I can offer you my opinion. I would definitely go with the Canon 2.8 glass. I use my Nikon 70-200 wide open at 2.8 and the bokeh is just awesome. You will also appreciate it when it comes to shooting fast shutter speeds in crappy light (rainy days, early AM or PM). The IS, similar to Nikon's VR, has its place. It does come in handy when shooting in low light. I can shoot at 1/20 of a sec at 200mm and get tack sharp images. One thing to remember with IS or VR though, and it is important to keep it in mind. It will not help with freezing action in low light. The only thing that works there is shutter speed, and that is where the 2.8 will be a huge advantage.

As far as third party lenses, they can be quite good. I know I really like my Tokina 12-24 on my D2x. It is very sharp and renders great colors and contrast. That is the only third party lens I own. I considered a Sigma when I was looking for a 300mm prime, but I went with the Nikon 300 2.8 because of its stellar reputation and outstanding image quality.

Also, search the web for reviews of the lenses you are thinking about. Take them for what they are worth, an opinion. Try to find sample images from the lens you want as well. THis can be helpful when it comes to seeing the capabilities (and disadvantages) of a particular lens.