The Onkyo DAP looks pretty useful if you want an all-in-one solution. Mojo as it is, is pretty fiddly if you want to use it on the move, because you need to juggle with USB cables.
However, I think since you already have a good DAP like NW-ZX2, buying the Onkyo DAP may not necessarily be an "upgrade", unless you care about balanced output or native DSD.
Sound quality aside, buying the Mojo is kind of like how Sony sells PHA-3 amp to go with NW-ZX2.
FYI, I never understood Sony's approach. They make a super expensive, very good sounding DAP like NW-ZX2 with a unique D/A conversion system (S-Master), and then they tell you, "if you want to drive powerful headphones, you will have to plug in PHA-3, which bypasses all the D/A and amp section from ZX2, and replace it with something pretty generic like ESS Sabre and some opamps"... It's bizarre.
I had to sell the ZX2 unfortunately, that's why I'll be using the Mojo with the S5. I also found it strange that Sony produced an under-powered player and pushed the PHA-3, but using easy to drive IEMs, it was fine for me.
By the way, will the quality of the OTG cable have an affect on SQ? Do you have any recommedations for cables please?