I've had a Vita for 2 years now, and have never regretted it. I've owned a 3DS since Animal Crossing and I hardly ever use it these days. I want to get more 3DS games, but it's always $35 - $40 a time. I get both heavily discounted games and freebies thanks to PS+. Yes the price of memory cards is ridiculous, but it's paid for itself by the savings I made because Sony actually has sales and discounts on a regular basis. Killzone is $9 this week in the US, and that is a FPS that is better than most console FPS games I've played. I thought I wanted Mario Kart, but Sonic All-Stars Racing Transformed is possibly a better game, and I got that free last month on PS+.
Someone earlier said Vita Uncharted was bad. That's very wrong, it's equally as good as the first Uncharted. The gyro-assisted aiming for pulling off headshots is wonderful, (Killzone has it too) and having played it three times for the platinum trophy I can assure you it's a worthy entry in the franchise. Now Ass Creed Liberation, that was a turkey.
Oh and for older libraries, the Vita wins hands down. All my old PSPgo purchases (yes I owned that silly thing) carried straight over. I recently got an NES game with my Nintendo rewards points and they look HORRIBLE on the 3DSXL screen. As in unplayably ugly. The screens are fine for the 1st party Nintendo style of course.
The Vita/PS4 synergy is a huge deal. Also the cross-buy policy on PSN. Recently I got a free copy of Flower for my Vita because 18 months ago I bought it for my PS3. No extra charge. As a portable indie machine it is also a hell of a device. Hotline Miami, Spelunky, and there is stuff like Rogue Legacy, Binding of Isaac, and proper Minecraft all coming too.
The 3DS is alright, got some great first party games. Just more variety, potential and value on Vita.