What I don't get is this. I have a 3DS. I really don't like it, the hardware, I mean. And there aren't many games I like for it. In other words, I'm completely confused as to why the 3DS is doing better. The DS I understood. The system was clearly more accessible, more portable and the library was phenomenal. None of that is true this time around except that the clamshell makes the 3DS more portable. Otherwise I think the Vita library kills the 3DS.
To me, the PSP was awesome because it got a ton of JRPGs, a ton of mid tier western RPGs (LOTR Tactics, D&D Tactics, Untold Legends 1&2, Dungeon Siege, X-men, Marvel Ultimate Alliance, Justice League, Silverfall), games like Test Drive Unlimited (free roam around Oahu), 2 Juiced games, Fast & Furious (Free roam around Tokyo, sorta), several NFS, Midnight Club, games like Grand Theft Auto, Gun Showdown (free roam Western), Star Wars Battlefront, Syphon Filter, etc.
Most of these games didn't quite work on the PSP, but they could on the Vita, it's got the power and the controls. But no one seems to even try to do these things on the Vita, only EA with NFS has really tried to put a console game on it. Even Uncharted/AC:L come up short compared to their console versions, though at least they put effort into the games. But not being a console gamer, I don't care about Drake or AC. Especially Drake.
Mind you, the 3DS isn't all that much better, but at least it gets first tier Nintendo games (which are big, to put it mildly), Monster Hunter, and a version of my favorite GBA game, Ultimate Card Games (confusingly named Classic Games Overload).
But speaking of the West, specifically, it's hard to understate how popular the FPS genre is. CoD: BLOPS2 and Halo 4 have just sold millions of copies. Yet the Vita has only 2 FPSes released in its first year, both by the same developer, both critically panned and arguably less ambitious than PSP FPSes (some of which managed 32 people online at once, Medal of Honor 2).
edit: Anyway, I like my Vita, I just wish it had more games I wanted to play on it. Even most of my PSP games don't work.