Since you are considering to have the tub for swinging, there are the neighbors to think about. If neighbors aren't a concern then it depends on where you live.
Personally no matter where I live I would have it inside. First, for the ease and comfort of just jumping in and out of it anytime you want whether or not there is a party going on. Then if there is a party then all the nakedness and sexiness is inside.
Expanding on the thought. Have one inside and one outside and I'll come over.
Wouldn't the lid being on the tub help with the humidity? Plus the room its in being big enough and proper ventilation would help keep the humidity down.