Sigma does make very good lenses. I own a 24-70 F2.8 EX and I'd definitely take that over the Nikon 28-70 F2.8. The Sigma will most likely outperform the Nikon or Canon if you get a good copy. If not, send the lens to Sigma and they'll calibrate it. The L series Lenses and Nikon lenses are built a little more sturdy though, and sometimes waterproof.
$400 > $1500 for me
