That's not a fair question, because we don't know know how important battery life and ease of USB charging is to you.
Based on battery life of over 24 hours and USB charging, and using it portable most often with portable phones, the D10 makes more sense. It sounds very similar to my RSA Predator amp, but with a better DAC. Later, when you can afford to swap opamps you can make the D10 sound close to the D4 and P-51 Mustang; but you can't make it as powerful as the D4 for when you move up to some harder to drive full size phones (HE-5, HD600 or HD800). The D10 USB DAC is extremely close to the detail and spaciousness of the D4 via USB, but the D10 via optical on my Mac can be slightly better than D4 if I use 24/96 files.
If you want a slightly better stock sound without swapping opamps, don't mind swapping between a pair of rechargeable 9v batteries often, and would like the extra 9v power for full size phones later, then you could go for the D4 which is also 1/2" smaller in length. Plus you can run it 24/7 off USB 5v with no worry about running out of power because you wont be needing the battery. And in 5v mode it will still be about as powerful as the D10 ever was. As a bonus, with a simple OPA1611A class-A biased opamp swap from HiFlight, you can make the D4 sound almost identical to the P-51 Mustang which is one of the best sounding portable amps I have heard. The D10 with opamp swap is close but still a little behind in treble extension and presence vs the D4 with or without opamp swap.