Help GAFers, help.
My last handheld wad the GBA. Should i get a 3DS with it's godlike library, or should i go with the Vita for Persona 4? I want long, long games with tons of replay value.
Depends on what games you like. But if you think that 3DS has a godlike library (as you said) and a Vita would be for Persona 4, then you basically answered the question yourself.
That said, I'm glad that I got the 3DS first, but I wouldn't want to miss my Vita either. Persona 4 Golden is a really fantastic game, but I just think that the 3DS has more to offer overall (especially considering P4G is "just" an enhanced port (it is a better game than P4, but still)), so I'd go after that and then get a Vita later down a road (or used right now, they can be pretty cheap (I got mine October of last year for 126,50€ incl. some games)). But again, it's all up to the games you want to play.
Edit: Oh, and yeah, there's some great DS stuff out there for you to discover. Vita can play PSP games, but only if you buy them off of PSN, so that might be a positive or a negative depending on your opinion about pricey memory cards and digital content.
On topic: I'm currently playing Mario Golf World Tour which I bought on Monday and I pop in Fire Emblem Awakening here and there to complete the support conversation library. I also work on finishing Bravely Default and Tales of the Abyss whenever I feel like it.