Why I don't want 69 is because it'd be horrible to be sixed up constantly.
But in 68, you'd have time, not every guy in there is gonna go gay, and the ones that do aren't necessarily going to force six upon you.
So I think 68 would give you more uhm.. Breathing room.
Also, I since I am attracted to men as well as women it's not a problem for me to have same-six-six.
Control vaults sound dreadfully boring though.
And experiement vaults never turn out well.