Even though I wouldn't bring 2 systems with me all the time, and the 3DS is always my first choice.
I think thats my main issue with the PSP/Vita atm is I like the 3DS too much lately. Zelda Link between worlds and Bravely Default are 2 of the last main games Ive played lately and I just got that version of Conception today as well.
I'm a big fan of both, and I swap back & forth between them. I've been playing the Vita recently with Danganronpa, and it looks like it may be my go-to for a while w/ some of the other games that have either just released or are about to. That said, I still want to get back to the 3DS to finish up Bravely Default. And a big draw to the 3DS to me is that it plays DS games, since I didn't really get into that system when it was big.
If you MADE me choose tho, I'd definitely go Vita - just has more of what I'm after. Tons of quick pick up & play games that are mostly free thanks to PS+, and tons of RPGs & quirky Japanese games. Heck, 4 more just got announced so it looks like it's picking up even more steam in that department.
I got an email from Nintendo saying that Super Mario 3 is finally up in their digital store for both Wii-U & 3DS. Which is cool, but $5 seems very steep to me for a digital copy of a 25 year old game.
I also think they take way too long to release these old games digitally, but that's a complaint I'd also level at Sony and pretty much every publisher out there. Why is there no digital Suikoden II / Xenosaga / .hack / Rule of Rose? These games all sell for big $$ on the secondary market (with the possible exception of the Xenosaga games, I haven't priced them lately), you would think the actual publishers would want a piece of that.