Not really into FPSs myself, especially not on a handheld.
I like action games and shooters too, but probably not the same you're into. I'm also really into sidescrollers. 3DS has the following games which all satisfy my needs:
Super Mario 3D Land
New Super Mario Bros. 2
Luigi's Mansion: Dark Moon
Liberation Maiden
Mutant Mudds
Mighty Switch Force! series
Kid Icarus: Uprising
Donkey Kong Country Returns 3D
Not a terrible lot of original content that catches my eye (and one of those is a port), and I border the lines between casual and hardcore gamer so gaming is a very leisurely, once-in-a-while thing for me, but you mentioned Vita's backcatalog, which the 3DS has as well through Virtual Console and DS backward compatibility.
Basically, for my needs, the 3DS meets them, and nothing on the Vita has caught my eye, and I'm not about to buy a handheld with better hardware just because it
might get games I'd like.
I guess what bugs me in all your 3DS vs. Vita posts is your attitude that "Come ON!!! VITA R00LZ! Get wit' da program, gaiz!" rubs me the wrong way, especially since you're so adamant to point out the inferior tech of the 3DS—which, again, means jack to me and to a lot of others, and really shouldn't matter and doesn't determine how good a game will be. Get some good games on that shit (even you admitted there's not much original Vita content) and I'll consider it.
Basically, you kinda come off as the guy at the beginning of this video.
Sega CD - Angry Video Game NerdIt's the '90s all over again.