New 3DS *without a doubt*, it's a console with an actual library and support. Unless you have a 3DS, you have all the reasons to get an N3DS and *no* reason at all to buy a Wii U. Look at the list of upcoming games - with Zelda pushed to 2016 and Starfox being an episodic remake of Lylat Wars it's a complete flatline. This is a no-brainer, don't get duped by fanboys who just want to keep dreaming. Get the Wii U next year from the bargain bin and buy a console with a future today.
He has a 3DS. His gaming PC's graphics card is playing his games adequately enough. Therefore Wii U seems like a decent decision. OP for the record, I would NOT touch a PS4 or XBOX One. The libraries are FAR too small if you have a gaming PC and every good release keeps getting pushed back. I'd actually recommend buying a new PC rig whenever the next generation of processors or graphics cards drop instead of a PS4 or XBOX One. The value just isn't there ATM whilst Dolphin is basically at 100% perfection of emulation for the Wii/gamecube and opens up a HUGE back catalogue.
I still think you should get a PS3. The library is absolutely nuts, the games are dirt cheap or buying a hacked one is easy as hell. And the catalogue is very very very good.
Otherwise, I'd get a Wii U but it depends if you like the games. I can sink infinite hours into Smash Bros. Mario Kart is a really fun fun fun game for friends and family. Then single player wise, Super Mario 3D world/Donkey Kong/Bayo1/Bayo2/Wind Waker HD (one of my favourite games of all time)/Wonderful101/Pikmin 3 are all pretty excellent. However, thats pretty much where the fun stops. This year, we have Xenoblade and Starfox too... splatoon is a genius concept but nintendo look like they will fuck it up. The future does NOT look great for the console in all honesty but I think its decent value with 10 or so good games with a few of them (smash bros + mario kart) having potential infinite hours of value. However, if they're not your scene... then stay away and wait for the next year or two as the next Nintendo console might be backwards compatible which makes the Wii U obsolete (i hope not and doubt it).
On the new 3DS front, I'd rather recommend a hacked PSP, Ds LITE and PS Vita for the price you'd pay for a new3DS/XL. It would give you access to a HUGE library of games including the 5-10 good vita games, every GBA game and every PSP + PSone game on the go. And since you have a 3DS, you're not missing out on anything.
For the record I have a new 3DS XL and although I do love it, it's only going to be worth it if you have a unplayed 3DS games you're itching to get into.