I like both of them. The Sigma 30mm is the sharper lens in the center over the 35/2 and I think for DX owners - and especially for D40, D40x and D60 owners as you get AF - it makes more sense.
In all honestly, you should probably Google this info as this question has been done to death on multiple forums, multiple times.
Also I'd consider the new Nikkor 35mm f/1.8 DX (not seen one, or used one), but it should be good, and definately worth a though for DX owners.
If you ended up with an S5 Pro, the 35/2 and S5 Pro is a very special combo.