The 3DS is the safer bet, you will find many amazing games on the system. My highest recommendations being Fire Emblem Awakening, A Link Between Worlds, Pokemon Omega Ruby/Alpha Sapphire/X and Y, and the remakes of Ocarina of Time and Majora's Mask. Smash 3DS is great, but if you are considering a Wii U, Smash U is the better version. Mario Kart 7 on 3DS is great too, but Mario Kart 8 on Wii U is excellent.
For the Wii U, the aforementioned Smash and Mario Kart are the best in their respective series. I HIGHLY recommend Splatoon, if you've wanted a very unique online shooter, Splatoon is the way to go. I adore that game. Aside from those three, Pikmin 3 is great if you have any interest in the series. Mario 3D World is pretty awesome, Mario Maker looks great, I haven't gotten it yet.
Wind Waker HD is the best way to play that game if you really liked the original, and you have Twilight Princess HD to look forward to. If you love Zelda, Hyrule Warriors is a lot of fun just for the crazy fan service alone. We also got confirmation that Zelda U will in fact come out on the Wii U, so you are guaranteed to get that as well.
Again, it's up to whether you want to take a safe route with the 3DS, or be risky with the Wii U. The Wii U is risky because the NX may come fairly soon, and will hopefully be backwards compatible. I really want more people to give the Wii U a chance, so I'll suggest you pick it, but I understand if you may feel apprehensive about it.