Depends what you like, but the Vita is very viable and very fun. It'll have more "serious" games, and remote play is cool if you have a PS4 or PS3.

Super Smash Bros 4, Persona Q announced for Japan and Monster Hunter 4 coming to the west some day? Hands down the 3DS. There's a game for every taste.

PS Vita.

Plus; there are some amazing deals on the hardware this holiday, Black Friday offers will be insane.

Get a 3DS, honestly it's not even a competition.

It depends. If you don't own a PS3 and, to a lesser extent, the 360, and don't plan on getting one or the other, then the Vita.

If you own a PS3 or 360, then get a 3DS XL. Don't even think of getting the regular 3DS.

The Vita has better games if not for the fact that just about all of them are playable on the consoles. However when it comes to exclusives, the 3DS wins. The Vita has some good ones, and is catching up, especially with this month having Ys and Tearaway as two more possibly good ones, but the 3DS overall has more.

*edit*

Multiconsole games between 3DS and Vita are superior though, so if you're thinking of a few of the 3DS/Vita multiconsole games and you're eventually getting both, then wait until you get a Vita to get those games.

Don't let people fool you though.  While the 3DS has more good exclusives, it's due to the fact that they have more in general.  The Vita is behind, but it's not too far behind the 3DS when it comes to exclusives worth owning.

You need a 3DS for Zelda: A link between worlds so...yeah.

3DS, and this is coming from someone who loves the Vita.

PlayStation VITA

Vita is great, but if I could have only one, it would definitely be the 3DS.