Both great camera's, I've not owned a D7100 but I do have a D610 and have had a D7k.
Full frame is much better in low light, gives generally better IQ and allows more latitude when working the RAW files in PP. The detail you can pull back out of the highlights and shadows with FF is astonishing.
If buying a DX camera feels like a compromise then just get the D610, else you'll only end up going FF in a few months anyway.