Jaywood2010 on 23 July 2013
DS, PSP, 3DS, and Vita
DS and PSP looks a little dated but still has a huge library.
3DS like the DS will likely have a bigger library than the Vita but the Vita will have raw power and to many that means better quality.
To me I would say just get both. There is no reason to miss out on games like pokemon X/Y and vice versa for games such as Persona 4.
If you only have enough money for one then it depends on what games you like better as they got way different libraries. Best is only an opinion so it is something you alone will have to decide on what has "the better library."
