I had the D5000 and also owns D5100.

After considering for an upgrade for almost a year, I finally pulled the trigger to upgrade to full frame (D750). I was looking at the D610 until I found out D750's introduction.

If you are passionate about photography( and able to afford it), moving to FX will be give you more options, features and much better lens to explore of your creativity.

It was not an easy decision as I have several DX lens and have been really pleased with my D5000 and D5100. My D5000 was stolen from me, but the replacement D5100 and DX lens found a home with my college daughter. She is exploring photography beyond her usual phone camera and P&S.

I saved up for the D750 and got two amazing zooms 28-300 and 16-35.

They are not cheap but well worth the investment.