The vita is for a more mature audience, while the 3ds is for a younger audience. But it all depends on what games you want to play. If you like games like mario then the 3ds is for you, but for everything else the vita is for you. I owned a 3ds for a long time but after I played on a friend's vita i liked it and bought my own. I found that media consumption is way better on the vita with the bigger and nicer screen. But it all comes down to what you like.
I have both but frankly, Vita games are generally a lot more fun to play. And trophies are nice. Not to mention the connected nature of the PSN so I can see who is playing what and where on my friends list. I also enjoy how if I wanted to get a Vita slim, all I have to do to download everygame I have already is factory reset my old Vita instead of filing a police report. And PS+ is awesome. And game pricing is actually reasonable. ..Why the hell is 3DS outselling Vita again?
I would go with the one that is closer to your needs now. Both with go down in price at some point and they're both quite affordable to have both now if you needed too. For me it would always come down to the games. If the games excite you than it could very well be worth it. You will need to find that out for yourself. I just ended up buying both and both have their merits, but it's all about the games for me.
Like any other console choice look at the libraries of each and see which appeals to you more.
I have Vita and 3DS. From a hardware standpoint they aren't on the same level. The Vita is a much niced piece of tech. Depending on what games come out I'll switch between the 2. The last few months I've been playing my 3DS, and just in the past couple weeks started putting a lot more time into the Vita again. Both are great.
I liked the PSP better than the DS and I thought that the new Monster Hunter games would be released on it (as was the case with the PSP). Was pretty disappointed when I read that Nintendo will get those from now on.
Turns out I should've bought the 3DS from the beginning... larger game library and lots of high quality games.
I think when it comes to handheld gaming, the PS Vita is the only way to go. I think the Vita is for more serious gamers, where the 3DS is just for the casuals. Way back when, I had a PSP as well as a DS. I always ended up taking my PSP with me, while my sister would take my DS.
I think both consoles have their place (I own both), but it all depends on whether or not you play handhelds with other people. Most of the Vita titles I own are single-player JRPGs, which look fantastic and handle great on the Vita's hardware. The 3DS has some great single-player games as well, but I find that most of the titles I play on it are geared towards multiplayer gameplay, such as Monster Hunter 4, Super Smash Bros., Animal Crossing, etc.