You shouldn't start threads like these, nothing good comes of them.
They are both very different systems aimed at different audiences; 3DS being the everyman device appealing to kids, gamers, ect... While the PSV is more of a "Hard Core" gamer handheld so to speak. And no, I'm not trying to imply the 3DS isn't for hard core gamers so put down those pitch forks.
Asking which one will be more successful in the long run is stupid as we know ow much of an iron grip Nintendo has on the hand held market. And really, who cares which one sells more? Sony isn't even looking to dethrone Nintendo, because even they know that such a task is almost laughable. They are simply offering a different experience and alternative, some decent competition that isn't just smart phones. As long as both systems continue to turn out more games and build themselves a quality library, both can co-exist (but haters gonna hate).
Really if the PSV wasn't around, all we would have is Nintendo, and smartphones...and smartphones are nothing more than cheap distractions than they are quality gaming experiences that you get with the 3DS and PSV.
And as far as launch line up is concerned, the Vita has had the best is recent memory regardless of what anyone has to say. I know people will just juxtapose their opinion on the issue believing that because there may not be games THEY are interested in at launch, it's not a good launch. Well from an objective point of view, it DID have more or less a good launch. Missing an RPG maybe, but 20+ games at launch, with something for everyone from action, shooter, racing, fighters, puzzle, ect...
The 3DS had a weak launch because it was push out too damn early. They could have withheld it until at the very least OoT was out, but they just didn't have that anchor game there at launch.
Both will do fine in the long run, but you don't really start seeing the real good titles until almost a year post launch. As long as both pump out quality software at a decent pace, they can both thrive.
But alas, there is always going to be these silly and petty competitions between platforms, like people need some damn reassurance that what they chose was the justified.
Haters gonna hate.