UE for sure. Impale i always cleared most mobs anyways, obviously less efficient, but how i set it up was that i basically am lazy and have veng on the num lock trick, so i hover my hand on shift and 1-4, and just hold right click down, and vault through aiming at whatever moves, while spamming vault for movement...works well. reason shift is that you can control when and where you stop to kill stuff, so its a game of shift, impale mobs, vault and shift to stand and impale mobs, vault, vault, shift...etc
for MS its a bit different, and faster. dont have to hold shift nearly as much, just hit vault, and 1 (for me, 1 is shadow power with the speed rune) and just keep moving, only right clicking for multi when needed, but now that its buffed, you don't fire off nearly as many multi's and use up nearly as much hatred as before, so you can focus the entire build more for speed, than raw dps, which means its way way faster and it clears entire rooms at a time, rather than impale that only can hit 3 targets at a time, and sometimes the stragglers get annoying or i guess just keep moving
MS clears everything, although this isnt super important in the long run, it feels better to kill everything than to only skip through to elites, but this is talking from a purely t13 speed run, obviously it changes when you do GR's, but for farming t10-t13, why not clear everything and all floors, more loot and xp, and gold for the goldwrap (which makes you pretty much invincible for anything that hits your armor, but look out for magic attacks)
You can tear through a t13 when geared decently (not stupid geared like 1500+ para, all augs and primals etc) under 2 min ish easily, which impale can maybe do but you'd need to play really well, and speed through, skip stuff, and hope for a good rift...where MS you simply enter and spam multi, and vault/speed run through, done. I've heard can even do sub 1:30 ish clears when geared, which i can see possible given how fast you can clear when you have avarice, hoarder, couple other things made for pure absolute 400% speed bonus, MS is a monster
so TLDR; MS is far superior for speed clearing t13 for dbs and keys, impale for higher GR's for gems imo.