Perfect information is hard to come by, I agree. But how is the Monk going to get perfect information? If you're saying the Monk's information about his target is better than the Wizard's (who often, though not always, has more accessible sources of information based purely on class features) - then you're already assuming an ambush scenario. If one class needs that kind of edge to pose a threat to the other, this discussion doesn't prove much. If information is equal (as it often is in actual game situation, at least ideally speaking), it's much more hit-and-miss.
But discussing stuff in a vacuum doesn't help assessing class balance. How about the popular "face five different scenarios, see how it goes"? I'll give it a shot, assuming high-level play:
Two regulars make one character each, one Monk, one Wizard. Let's say level 15. But we can expand the challenge to include each one at 10th level as well as 20th level as well, if you like.
Each player picks one of the following seven scenarios to disallow, so five scenarios remain.
1. The Monk is a treacherous party member. He knows the Wizard's tricks intimately, including his defenses. He is implicitly trusted by the Wizard and has easy access to his stronghold (even an extraplanar one). The Monk is able to take his time to do the deed whenever, and is ready to devote his all to it.
2. The Wizard knows exactly what's coming, the Monk-of-murderous-intentions has unwittingly been ratted out to him early on, and the Wizard has done excessive research using all sources available to him on his modus operandi, power, resources etc. However, the Monk can pick the place and time to strike. Assume a paranoid Wizard, though.
3. A Monk is suddenly blackmailed to go after an equal-level Wizard he's never heard of (but neither has the Wizard ever heard of him). He has 1 hour to give it his best shot, before the Mafia kill his whole family. Assume the Wizard is on a quest to save the world, which takes precedence over all else, and doesn't suspect the Mafia to be out for his blood at all.
4. A Monk is suddenly blackmailed to go after an equal-level Wizard he's never heard of (but neither has the Wizard ever heard of him). He has 1 year to give it his best shot, before the Mafia kill his whole family. The Wizard is a great hater of Monks and Mafia, and generally paranoid where those guys are concerned. He has no reason to suspect an assassination attempt specifically by this Monk or this Mafia clan, though.
5. At the end of a long day of adventuring, a Monk and a Wizard from two different continents are unexpectedly and forcibly teleported to an open, high-domed, teleportation-and-summoning-proof arena. They're told only one of them will leave the arena alive. Also, both are completely stripped of gear.
6. At the end of a long day of adventuring, a Monk and a Wizard from two different continents are unexpectedly and forcibly teleported to an open, high-domed, teleportation-and-summoning-proof arena. They're told only one of them will leave the arena alive. Also, both are given exactly five level-appropriate magic items of their choosing.
7. Right before they embark on a long day of adventuring, a Monk and a Wizard from two different continents are unexpectedly and forcibly teleported to an open, high-domed, teleportation-and-summoning-proof arena. The Monk gets a Wizard bodyguard two levels below himself. The Wizard gets a Monk bodyguard two levels below himself. Each tag team has to kill the whole enemy team to win. If both sides suffer casualties, the fight is considered a draw.
What do you think?