That's an easy one. It terms of all round usability the Nikon 35mm f1.8 is easily the best bang for buck lens you can get for DX.
The 50mm is very similar, but more suited to portraits etc, and a bit tight for general use. You could probably pick up both the 35mm 1.8 and the 50mm 1.8 afs for under £350. Both are great lenses.