Both of these systems are fairly new and their library of games are still expanding. I have a VITA and a 3DS and PERSONALLY (don't bash me) I like my vita more, I believe that the Vita is more suited to adult gamers or the more than casual gamer whereas the 3DS is suited for "kiddos" as andrew said. Don't get me wrong though, there are a lot of games on the 3DS that are suited for gamers like KI and KH3D to name a few. Still, it boils down to that. Also, something that drove me to buy a vita was how I wouldn't need to worry about cartridges because I get my games digitally. If I needed a game out of my PSN region, I'll get the game on the card because there is no region locking on the vita.

THIS IS JUST MY OPINION, PLEASE DON'T BASH ME.