I would also recommend the 35mm f/1.8DX. It is an excellent lens and a bargain. If you can find a good used copy, all the better.
However, I would not think you would want to take it with you to FX, once you go there. While it will work in DX Crop mode on a FX body, that kinda defeats the purpose of going to FX, doesn't it?
If you are planning one switching to FX within the next 6 to 12 months, them maybe you should get one of the 50mms that will AF with your camera. Either of the Nikon G lenses will work, or the Sigma 50mm f/1.4, which is arguably the best of the 3.
Otherwise, get the 35, and once you go to FX you can either sell it, or keep it as part of your back-up kit with the D5100.