Overall, Mumor is better imo. Her melee and ws damage is fine for a trust and additionally uses Saber Dance. She also stuns pretty well. Uka isn't really a reliable healer, so you would still summon a healer if you need anyway.
I don't think you'd generally want to use a WHM trust + Uka. More practically, I think she works as a niche healer.
She seems to work well if you, or another actual player in the party are on a healer job. Uka seems to be fast on curing people back from major damage, but doesn't usually top them off, and doesn't have Haste to use. The trust AI doesn't really cooperate, but a player can account for her taking care of some healing.
The other angle is that WHM trusts burn through their MP like nobody's business, but Uka will keep generating TP. So, in situations where you can't dismiss/resummon trusts, she might prove to be particularly useful.
Mumor is certainly the more generally useful though, since she's got a decent combination of DD and utility via steps/sambas/Violent Flourish.