Two years later....
Seriously, if you have to pick between buying a 3DS and a Vita, definitely buy the Vita. The library exists now, good games are available with more on the horizon.
If you can hunt down an OLED model Vita, buy it! In a side by side comparisson the OLED screen is mind blowingly better looking that the revised Vita, which has an LCD screen. And if hand held gaming isn't your thing, Sony is releasing the VitaTV, a console version of the Vita with HDMI output, stateside in October at your choice of $100 without a controller (uses PS3 and PS4 controllers) or $130 with controller. It's an awesome time to buy Vita as it's future is very bright.
Now There are no Limits -- SEGA