D600 with D7100 as Back up

I think a lot of us fall into your situation by coming from higher end DX cameras.  It isn't a bad compromise in my opinion.  DX gets some extra reach with higher resolution/speed and FX covers the rest of the gamut.  I like your thinking as to the combo as well.  Might want to wait before you give up on Pentax though and see what they come out with about April if the rumour is correct.  We may all be surprised by them and you already seem to have some glass.  Also waiting allows any QC to work through the system.