It depends if you are really going to use the longer lens.
The 5100 has a slightly better sensor and some more controls, but you will never really see any quality difference between the two. The D7000 is a more logical step up - controls, construction, capabilities etc, but with the 5100 sensor.
If you're learning you have a very good set-up that can produce top grade photos.
Should you progress in photography you can always buy a larger/more sophisticated camera and keep the 3100 (with, say a 35/1.8 or a 40/2.
as a compact, convenient camera. My D40 (roughly the size of the 3100) with the 35/1.8 is really compact and light - a pleasure to carry round, and with a doof viewfinder to take photos on the run.