The DX equivalent is rather 16mm at f5.6. In both cases - FX and DX - a 36" print would be diffraction limited, a 24" print would not be. Anyway, the practical advantage of DX comes on the supertelephoto side. I'm currently shooting a particular red morph screech owl, and can't get closer than 50'. With the D7100 I'll shoot with my 500 f4 and TC-17E at f7.1 using the 1.3x crop mode. This gives me the equivalent of a 1600mm lens shot at f14 on FX. Well, as it happens that could just be done with the new 800 f5.6 and TC-20III converter, but the viewing would be really dark (f11 versus f6.7 on the D7100) and I'd be restricted to manual focus.
Anyway, in theory FX is a superset of DX - anything you can do on DX can be done on FX. In practice DX has a real reach advantage at supertelephoto. FX has the theoretical and practical advantage of allowing for a one stop shallower DOF, should your photographic practice require it.