Quote:
Originally Posted by
Eric Samonte 
Some would say go for the 2.8 but I think the weight will turn me off, eventually me not using it as much. Besides, I can up my ISO if ever I need to use it indoors or in low light.
There can *NEVER* be enough light or ISO. 
Ok, not exactly always true, but if you plan to shoot in low light a lot (w/out strobes), then you can use *ALL* the light and ISO sensitivity you can muster. Of course, as you open up the lens, you will also have to deal w/ shallow DoF, which can become an issue, though it probably won't be too shallow most times at f/2.8, if you're using a cropped body. If anything, you might find that f/4 is not shallow enough for many cases (whether there's enough light or not).
But yeah, the weight can become an issue, especially if you're already sensitive to that. I haven't been lugging my Sigma 70-200 f/2.8 in a long time either since adding the Nikon 70-300 VR, but then again, my old Sigma didn't come w/ IS and didn't cost me nearly as much as the current going rate for the Nikon/Canon 70/80-200 f/2.8 IS (nor non-IS) either. Plus I can always opt to bring my (optically better) Nikon 180 f/2.8 instead, if I think I need f/2.8 at the long end -- that combined w/ the 70-300 VR probably only weigh a little more than the Sigma alone although the combo requires more bag space to lug...
_Man_