I'm guessing Ki-Adi-Mundi would be a decent adversry being a Jedi master, but he got owned by Grievous and killed by a few blasts from some Clone Troopers. Meanwhile, Barriss was a frontline Jedi healer and engaged in hostile territories such as Ansion before AOTC. She strikes me as being a better fighter, plus she is reputed to be extremely acrobatic, even for a jedi and a Soresu practitioner.
I agree with Janus. And Ki never struck me as being the greatest fighter, espescially after seeing him get bested by the General in five-on-one combat. But the time he fell in ROTS was sad, and he did manage to take out four or five clones before he fell to a dozen of them. Barris is a Soresu practitioner, and so she could probably fend off and eventually defeat Ki. But he is pretty good nonetheless.