I have the 3DS OOT on my to-play list for the next year. I'll be my first time playing the 3DS version.
I played ZombiU on the Wii U and it made great use of the second screen. I thoroughly enjoyed it. Very atmospheric. You really feel as if you're in a zombie-infested London. Good story and a constant feeling of threat. If you do decide to play it, you should know that you'll die a lot. It's part of the game. I also played Resident Evil: Revelations on the Wii U. ZombiU is definitely the more Survival Horror of the two. If only it had a Rob Zombie soundtrack...