First, greetings to you all. Its good to be back, great the forums are back up
As for the original question, I believe Monks are probably the better character. I've said this before, but the way I see it, if you have a half-decent mage in your party (Edwin...), it is usually unnecessary to dual a Kensai to mage unless you cast MANY spells in battle. By the time ToB rolls around for the K/M, his melee skills will be withering (I know, thats my chars predicament), while he is gaining mage skills that are already covered well by Edwin. At this same time I would imagine that a Monk would be coming into his own, and totally owning all comers.
A more interesting debate would be just a plain Kensai vs. Monk.. I'm not sure, at lvl 20 both would be very powerful, off the top of my head I'd probably wager on the Kensai. The Kensai would be wielding powerful weapons and be receiving +6 dmg and +6 tohit, just as a class bonus at lvl 20, not considering strength bonuses. And a -5 to speed. Its a toss-up, they would both rock at high level.