It's a tough call whether or not it is worth double the price or not. If you are just getting into audio, I believe that the Zero would be a better idea because then you could put your left over money toward a much better amp. Zero also has a preamp if you need that at all, the Cube does not (well, cannot control volume with dial when powered monitors are plugged into it anyway). The amplifier in the Cube is actually decent. I have been listening with my DT880s and they sound great. Will wait at least a few more days to actually tell people how good the amp in it is compared to my M^3 though.
I might sell the Zero, I am not sure yet. Might keep it just because an all in one device can be very handy at times.