It depends on what you want/need. The nanoKey will be your best bet for easy interface with computer/DAW/Ableton, etc. And, it has more of an "actual keyboard" (actually, neither the nano nor the Volca series have what a keyboard-player would call a "keyboard").
I am a big fan of the Volcas, however (I have the Volca Bass and Volca Keys, and am very interested in the Sample), and am a big fan of the small but powerful Korg boxes in general (I also have the Monotribe, the Monotron, the miniKP Kaoss pad, the Kaossilator, and the Kaossilator Pro). You will be able to work with other gear (drum machines, samplers, computers, etc.) via MIDI, and if you are using other similar Korg devices, there is their proprietary sync ports. So there is some "interfaceability." But hooking it up may seem a little arcane if you're not into hardware much. I think you will have, in a sense, a broader selection of sounds available with VSTs, but then again, if you want unique sounds that others won't necessarily be using, and you want real hands-on control (or at least a gateway into the world of hands-on control), then the Volca FM might be the way to go.
GJ