I'd suggest a combination, since I really just wanna know what is ON the higher up floors (and the highest) at this point XD I also love the Guardians plot, though.
I guess you could have at least one mission per set of quests in which you do both? Like, one mission to a new floor for two players, another mission to a guardian for another two players
They sometimes say, "the place where I am right now was circled on a map for me"... Unfortunately, I kind of suck at orienteering.