I am a bit of a lens research nut myself, and I am a Nikon shooter, but the only Sigma lens I have actually used is the 10-20mm which is obviously a bit of a specialty lens. But the conclusion I've come to on the pricing of various lenses is that 2x price is rarely, if ever, twice as good- it just comes down to a question of how much you want/need the extra features you may be gaining.

I agree with Barefoot's comment about the ISO button being your friend. Most of the current crop of cameras perform fairly well at higher ISOs (to get that speed as an alternative to max aperture) and what noise remains the various post processing noise reduction tools can take care of quite nicely. So, although I like fast glass too, it's always significantly more expensive (or forces you to go with 3rd party brands- which may not be bad, my personal opinion and experience aside), but also bigger and heavier, AND sometimes these pricey lenses are still not even very sharp wide open... I know this is physics to some degree and any lens wide open will be equally challenged despite what the max aperture is, but it always strikes me as odd that I'm paying for a 2.8 but reviews say it's better stopped down to f/4.

My vote would go to the Nikon 16-85, if size/weight and budget are major concerns... it's a very highly rated lens with a great focal range. If I'm not mistaken it even has VR which "gains" some speed in more static scenes....

Or, if you can justify the Nikon 17-55/2.8, JAS loves hers and by all accounts it's a great lens.

Hopefully others here can chime in with more experience on the Sigma lens you are considering. But if you decide to go 3rd party just make sure the store has a good return/exchange policy, another copy in stock, and get ready to shoot some brick walls.

I know it's pretty tough to make those decisions in store, on the spot, so good luck!!