If you like both games, get both games, Id make a strong arguement to say that they are the 2 best games out on the 3DS right now. If you HAVE to get one, then get monster hunter. Much more gameplay, its a steep learning curve for sure but once you get the hang of it then it becomes incredably addictive and there is also a nice incentive to get more gear/diff weapons and the graphics are beautiful.
I do agree with the poster above me tho, both have demo's and both demo's show the core gameplay you will be experiencing in each game so if you find you enjoy one more than the other, get that game but if you enjoy both, get monster hunter.