To provide my "credentials" for the question at hand, I have had a Vita since launch and I've been actively borrowing (and returning, of course) my friend's 3DS for the past year-and-a-half.
I really like the 3DS. The games it has are generally quite fun, and it has a pretty vast variety. The quality of the first-party stuff is really high, and the 3D works fairly decently beyond a simple novelty
But, also, I really like the Vita. I've played more games through on the Vita, and have generally had a lot more interesting and "bigger" experiences on it. Where I would regularly play 20 - 40 minute sessions on the 3DS, I have absolutely had multiple 4+ hour sessions on the Vita. It feels more like a powerhouse.
I think the fact that you have PS+ is actually quite huge. About half of my library is games I've gotten for free on PS+ - and I haven't even played through all of them, yet. The ones I have are great, and I gladly would have paid for them (wipeout, gravity rush, thomas was alone, and a few others). Also, I do plan on getting a PS4, so for me it makes sense to stay with the Vita.
tl;dr - I personally recommend the Vita a little above the 3DS, but both systems are great at what they're aiming to do.