League of Legends (LoL) Question: AP offtank Cho'gath build: Nashor's Tooth or Wit's End?

I'd say if you're going full AP cho, don't build any of them. You need to be alive too much time to actually get to people and do only a couple of autos. If you're going offtank I guess wit's end is better, just because nashor's works better in AP heavy champions, and cho lacks the mobility/range to actually make optimal use of nashor's when building full AP.

You're right in thinking it's situational. I'd say that Wit's End is the safer bet of the two. It's cheaper, has flat damage on-hit, MR shred, and MR.

Nashor's Tooth is nice but is difficult to use effectively.. it makes you want to build more AP but if you're trying to go more tank, you can't; and if you're building more AP, you won't be in the middle of a fight throwing AAs too much.

There are times either could be better but for a general discussion, Wit's End is safer than Nashor's Tooth.

Wit's end is better if you have another AP Carry on your team, since you're shredding their MR with your autos, you also would get wit's end if you tend to be auto attacking a lot. The thing about wit's end it that auto attacks shred their MR, not abillities, so by you doing your full combo (Q, W, R) you aren't utilizing the passive well, and you would just be wasting time trying to reach your enemy to auto attack them. Gnashors tooth I would say is a little better because you utilize the AP more than Wit's end's passive, since it helps with your combo, and it also helps with your auto attacks, just like Wit's end too. Also since you said you were going AP offtank, you aren't really tanky enough to slug it out with the enemy team just to get some autos off, you're better off just going full AP with Gnashors, or Full tank, with Wit's End.