Hey there
I think that the sound is different but either is a different option to each other, especially since X7mkii has double microSD card slots but also costs more.
The sonic performance is a hair better on DX150 if you're running it with AMP5 vs X7mkii with its AMP5, but DX150 has one microSD slot, thing which you need to take into account.
The build quality is great on both, but with DX150 the body and the AMP module combines a little smoother.
The software is pretty different, there's less complaining about DX150 from the overall public, although I never ran into any issues with either. DX150 has Lurker's mod, which is really nice.
I need to make a very in-depth comparison during my DX150 review to help with this question.
To be honest, for my typical usage, which is listening to music from my collection, both are great devices. The rule of thumb is: FiiO is more neutral out of the box for sure, DX150 is really smooth and warm with its default AMP6, so if you prefer a warm and smooth sound out-of-the-box, then DX150 is warmer and smoother, where X7mkii with its AM03A out of the box is fairly neutral. On the other hand, since both are modular, you can always change the sonic signature with subsequent AMP upgrades.
The display on DX150 is a bit larger and has more natural colors, and higher maximum brightness, but this is from using it daily and noticing a difference, not from using a color checker thingy.
The GUI and device design are very subjective things, so I can't comment on those.