It would depend on how you'd define each.
If a behemoth represents a larger, much more powerful enemy with special powers or even just greatly increased stats (well above and beyond what anything else can achieve) then it could be a useful end game adversary. Maybe the behemoth was the result of some early attempt at a cure that went horribly wrong? Maybe killing one gives you a schematic/book for a some powerful weapon or armor? Maybe they aren't meant to be killed and instead represent something you have to run from... flee or die, seek daylight as your only escape.
If zip lines have to be constructed with some forethought and effort and can be destroyed then I don't see a problem with them. You set up the high point, travel to the low point and construct it. Now you can travel in one direction from point A to point B. What possible harm to game play could happen from that? But if they're more like the automatic thing of Far Cry then no thank you.
Long term, if the game mechanics could be fixed to allow larger enemies like the behemoth then I can see potential in the modding realm. For zip lines they add some flavor to base construction and possibly to city navigation (and I'm sure they could be modded for more fun).
Personally, I think I'd rather have what the behemoth would add to the game engine, especially if it would allow enemies of nearly any size. Giant spider hordes here we come! My vote: behemoth