Depends really, but nevertheless, you can't go wrong with any of them.
I own both, have a blast with both. Moreso with my vita tho just because, like someone said above, there's a lot of oddball, new licenses, and stuff I didn' get to play before that plays quite well on the Viturr. Also, it's got a shitload of announcement recently for 2014, which will make it a very interesting platform. Plus, PSN allows you to buy most psp titles digitally and some ps1 games. And if you have a ps4, well you get to have remote play.
And if you like Indies, well it's getting a ton of that too
It's major downsides are the price of memory cards which are still outrageous (although the console itself got to a reasonable price), some pricing on the psn is a bit high too but they often do interesting sales, and PS+ offers nice advantages.
And well, while it's getting great support from japan and decent support from europe with wipeout, killzone M, and tearaway, america seems to be completely ignoring this thing which is a damn shame. So if you're not too much into either niche (but good) titles and japanese games, you might find it tough. Although I wasn't too at first but by curiostiy and recommandation, tried a few and found myself enjoying them a lot.
3Ds though is also a very strong choice too. The sheer slew of titles it got in 2013, and still getting some in 2014, makes it one of the most powerful platforms this gen in terms of software. While some of these title are still a bit formulaic, the amount of 'em compensates so much for it, it's crazy. And it's backwards compatible with the DS, so hey, you get to experience the MASSIVE library of the DS as well as the 3ds.
It's major downsides are for me the hardware and software of the actual console. Especially after playing the vita, the lack of almost any form of multitasking as well as the still quite weak online solution makes the 3ds still behind in terms of actual OS design and hardware capability. And I often find myself not utilizing the 3d all that much as it either fatigues my eyes or even reduces the game's fps (like in Pokemon). The lack of a right stick (although you can have an additional one, although for a fee) makes it quite annoying for certain games.