I owned the U12T and would choose the M9 over the U12T any day/every time! The U12T sounds bland, thin, sibilance next to the M9.I was going to ask, price aside, which is the better IEM between M9 and U12t for you? I suspect it'll still be the U12t, but possibly depends on preference. They U12t is still the pinnacle of all-BA IEMs for my taste, the most balanced between musical and technical - and excellent at both. And that's from someone who swears by DD bass.
The M9 would absolutely destroy the U12T in details (later), imaging, separation, imaging, timbre, image broadness, bass size, cohesion, vocal weight and focus.
With that said, the U12T is no doubt having better resolution, bass speed, soundstage depth and a unique sense of dynamic. Speaking of dynamic, despite being very satisfying, it is at times too much leaving some micro details to be completely masked by that punchy dynamism thats why the M9 pulls out more details to me, everything is simply there, not hidden or masked by anything.
To be honest, the choice heavily depends on personal taste but if what you're looking for are the strength of the M9 then the U12T would not serve you well.