I had M8 and DX300 together and choose M8. I don't pocket my DAP much so take it with a grain of salt...
M8 is probably less likely to press the volume/power and play/fwd/back. I remember frequently pressing the volume/play/fwd/back button by accident on DX300 with the leather case. M8 is heavier, slightly thicker, and runs hotter, so I hope you have a vented jeans pocket.
Sound-wise, DX300 has the iBasso signature spacious treble sparkle, flatter bass, and overall more neutral than M8. If you are in-tune with that signature, you might find M8 warmer depending on your choice of IEM/HP. However, if you give it some time, I think M8 has a pitch-black background and a very deep detail within a warmish analog sound. There's an exceptional clarity around 60-120Hz bass region that gives you nice clarity for double bass/bass guitar lines. This has been the easiest DAP to pick up the pitch of the bass notes. Subbass probably doesn't come across as much as DX300. I had fatigue with DX300's lower treble. I listen to M8 day and night, no problem.
You will lose simultaneous access to 2.5/3.5/4.4. Also, don't expect DX300 level UI experience. That is the most phone-like experience.
My $0.02