Of the two, I think I used Monster Hunter more. MH is like a free battle game where there's no health bars, just you and your monster. It's pretty easy to get the hang of it, and many people here play it so you're set for online play.
On the other hand, Red Steel 2 is also a neat game. With wii motion plus, it makes the sword fighting quite realistic. The graphics are fun and cartoony, which gives the game a neat feel. As far as I've gotten, the story is pretty cool and there are good bosses.
I guess it's up to you, but both games will be good investments. Just remember to get a classic controller (pro if you want) if you pick up MH3, and wii motion plus if you pick up Red Steel 2.