I used to have a Sigma 75-300 zoom long ago and a 24/2.8 that I bought new in the 1980s and sold only recently. Optically, they're not bad for the price, particularly in the area of wide lenses, but build quality is lighter than most OEM lenses. In their early days, they had real problems with this, and that has damaged their reputation to this day (you'll see references all over the net to "SIGnificant MAlfunctions"), but I don't think they've really had those sorts of problems since around 1982. I'd avoid the Sigma long teles and tele-zooms, but the 24mm or 15mm are probably a good bet, if you can't justify the cost of the Nikon.