hm, I'm not gonna vote in the poll and say both.
if you're looking for something right now, I'd say Minecraft. I just got it recently, and I can testafy, it's a brilliant game.
however, the 3DS is not without it's merits. I'm probably gonna wait for a price drop on the 3DS, seeing as how the only games out right now that interest me are Ocarina and Street Fighter. once Super Mario, Mario Kart, Kid Icarus, Star Fox, MGS, and all the other kickass titles come out for the 3DS, then I may pick one up.
then again, Minecraft is only $21...
I don't wanna come off like a rich prick... but if you have the time and money... why not buy both? I mean, it's not like you're deciding between two consoles, you COULD get both... just sayin...