The 21 will require many more upgrades to deal with the power for a very long time. I'd go with the 3.3. Installing 10 slipper pegs (or the 3.3 slipper/spur conversion), the FOC inside the trans is about all you would need to do for durability. Then a new pipe to let it breath.
Thats were you are kinda wrong. You do still need a motor mount. Plus, You will have to upgrade your drive shafts to CVD's because it will twist them to a snapping point. Your tranny will have to have a FOC and probably be better off with the 3.3 clutch slipper set up.
I've had way to many problems with the 3.3, mostly its the front and rear bearing that craps out ALL THE TIME! its kinda hard to tune also. and the starter thing is weird because it lets fuel leak to the OWB and you have to clean it all the time, its a pretty bad design. I've taken my trx3.3 apart 3-4 times this year. the O.S. motors have a sealed bearing in the front so you dont have to clean the OWB all the time. the O.S. also has a nicer carb, and a nicer drive shaft that allows more fuel air mix to the engine. so go with the O.S. their spendy, but worth it. the trx 3.3 is kinda a cheap thing.