Look at the games out now and pick the one you will buy he games for, both handhelds are coming to the end of there life, the vita never really took off but has some great games and if you have a ps4 its the most expensive controller ever! I got a new slim one at Christmas in orange its very nice but I have ps plus with lots of free indie games.
The 3ds again is at the end as Nintendo will and should announce the NX this E3 which rumour has it its a main base unit for the home and a portable which will replace the 3ds but will be the controller for the NX.
Both have great games out just see what you fancy, JRPGS can be had for both.