Diarmuid has better demonstrated skill. Ushi is more versatile, she can re-arrange the battlefield to build and has a sonic attack. She can also make a shadow Benkei to block attacks though Diarmuid would probably just cut through that. Granted she didn't use any of these against Gorgon but I suppose none of them would've really been helpful and it may just support my suspicion they don't want to show anything attack animations already don't.
I'd back Diarmuid in overall CQC although the attack Ushi used on Gorgon could be a problem. It was most like Usumidori which is stated to be hard to avoid and he obviously can't tank or block it. Still Diarmuid does have that skill that makes his opponents mess up so I see Ushi engaging in CQC too long and taking an unhealable wound.