Rogue does a lot more damage, but monks come with a little more utility, for instance you could solo/duo (especially duo) with a monk MUCH easier than a rogue. The only thing that keeps rogues exciting at all is finding ways to make bigger numbers (raid-game, anyway). I could do things with my scummy monk alt when it was poorly geared that I couldn't do with my tier oh-my-god rogue without severe buff and clicky abuse.
If you just wanna raid, rogues are great because there's no over-lap on a raid roster, 3 rogues is fine. Monks CAN stack just fine on raids, but a lot of raids prefer only one in most situations because a rogue will put out more DPS.
If you want to farm, nothing wrong with a monk/shm or monk/dru combo. Don't expect near the mileage out of a rogue in the same setting. Farming as a rogue means groups or painful progress, but as a rogue you also don't spend near the cash nearly as fast as other classes. A rogue can take a Charm of The Nemesis clear to nearly end-game without catching any flak at all, where other classes benefit HUGELY from having the best charm you can get as soon as you can get it.
(me bragging) when I still played I dominated (#1 DPS on the server!) the DPS charts with a halfling rogue with a charm of the nemesis and a searfire aug. Throwing down the gauntlet here, yes better than the #1 rogue on the class top 5, yes better than team germany, and in more than a few situations, better than the other 17 toons mashed together on thaz raids (suck it folks)
Of course I'm now in-game-retired so nobody can ever prove me wrong (suck it!)
Also something to consider, like I said raid slots are easier to come by with a rogue, but if you manage to nail down the monk spot on a raid, you usually get tank-like loot priority.
tl;dr: If you just wanna hurt things the best in a raid and most groups (and spend most of your solo life hoping green cons don't splat you): rogue. If you want to hurt things really well and be able to do amazing things on the side (like farm things without being godzilla-in-smurf-village-tier or tank groups): monk