I’m forgetting how U12T stages, because I don’t reach for any IEMs besides MkII customs, if that tells you anything. I don’t recall it being overly big or too small, but I do recall imaging being excellent. They were the first IEM to finally dethrone Campfire Solaris for me.
U12T is probably the best fit if your ideal is a neutral, natural sound–a reference type experience where everything is presented in a clean, well-balanced manner. It’s not at all “dry” or “clinical” or any of the terms people use to describe tunings that favor neutrality to the point of sucking the life out of music. Just a very clean window into music.
OG MEST probably has the most holographic presentation, but it also has some (minor) issues. It doesn’t play well with all music–the first track I played on mine was “Them Bones” by Alice in Chains, and that turned out to be kind of a terrible track–the mids lack energy, making that song far less energetic than it ought to be–it was boring. Still, other tracks can be amazing; MEST made a name for itself for good reason.
MkII upgraded the bone conduction to a dual-sided, full range driver. Honestly, that’s enough to make OG MEST obsolete for me, because MkII does play well with everything I’ve thrown at it. Although many have said they don’t get the “crazy 3D thing” as much on MkII, it’s definitely still there, and definitely still wild with certain tracks. I have a whole playlist of stuff that shows off MkII staging plus the subwoofer-like bass.
Personally, I found the MkII universals a little darker than I prefer, but my customs have a more consistent presentation (almost certainly because of more consistent fit) and a bit better treble performance. They’re “close enough” to perfect.
If holographic staging is your priority, MkII customs all the way. If you don’t want to jump straight into customs, get the unis–you can always have UM reshell them later.
Unfortunately, Sedna won’t seal for me.
so I’ll have to wait till next year 
