I'm not loyal to any particular gaming company, but I'm sure the 3 companies that make these 3 handheld portable systems are competitors to one another and I'd like to know if any one of them has a leg-up on the other 2.
I'll be honest, I don't like the name of the Vita, but it has some connection with the ps3/ps4, so I assume I'll probably have to get those consoles to get the most out of it.
For the Wii U, I'm well aware it has been struggling, but it looks pretty neat for a handheld, since it has wi-fi and HDMI capabilities. Still need to do more research on it, though.
As for the 3DS, might be a little premature to ask about getting it, since I don't even have a 3dtv, or the glasses. I doubt it comes with the expensive glasses, but I could probably snag a copy from my local theater, but they are usually cheap and flimsy. I'm certain the 3ds doesn't include a 3dtv in its box, though.
this screenshot is for warcraft 3 players: http://img10.imageshack.us/img10/5835/masscritters.jpg
Anyways, I would recommend a 3DS as it is the best one. The Wii U gamepad is indeed fun but you can't carry it everywhere unlike the 3DS and the Vita is severly lacking on support so the logical decision would be to get a 3DS.
Preferably a 3DS XL as I have found that it is much better than the original one.