My first reaction was mamba, and I actually thought that I was going to be in the majority. I don't know much of black mambas, but most of the snakes I've encountered first hand tend to not want to be anywhere near you and will almost always try to avoid biting you. But on further research it seems they are rather territorial. Even so, I do think they'd be more likely to want to hide than stay in open spaces. But that's a double edged sword, since they'll probably end up in the most random nooks and crannies then bite your head off (or at least try) if you stumble upon them.
Realistically though, regardless of which one I choose, I'm going to try and stay as far away as possible. If it's snakes, I'll probably climb onto a table or something, maybe up on top of some shelves in a grocery store. If it's a gorilla, I'd probably try to hide somewhere hard to access. On top of a lift or in a walk in fridge as others have suggested seem like good bets. I don't really trust either to not try and kill me given the right circumstances
I was gonna say. The gorilla is a 800lb. problem that requires stealth, strength, and fast thinking to deal with. You'd need a smarter plan than "lure him into Banana Republic and close the security gate with him inside." But you will need a plan.
Also, accidentally make eye contact or show your teeth to that ape and you're dead; like your shoes came off, dead. Can't say the same for the snakes.