I haven't played both into the very late levels but I will still give my opinon hehe. In the fun factor and the roleplaying factor monks get a good score. The idea of playing a monk is pretty cool. They are very powerful and pretty well rounded.
The thing that I think throws the balance in favor of the kensai/mage is the spells (heh well duh). I spent the extra time (and regreted doing it up until I unlocked my mage spells) of going to level 12 kensai before dualing. So with level 12 kensai abilities my melee is very impressive. the thing that makes this char so great is the cool spells like sequencers and contingencies though. With a contingency to say dispell magic when you lose control of your char or something that helps alot. Then throw in the cool abilities of of some of the weapons you have available to you also. Celestial fury is nice. Daystar is nice. equalizer,pixie prick. A monks best feature is also a limitation. Sure you could use a sword as a monk but why? I really enjoy doing improved haste and turning into a lawnmower with my kensai/mage. Oh the enemy cast stoneskin,improved mantle etc? just bust out some spells on him and then hack him up
still though there is alot to be said for the roleplaying. I can't say don't play a monk even though I love kensai/mage. My monk wsa a blast too.