Do you want to go to pool parties, or just hang by the pool? If pool parties are factored in, I would suggest not staying at PH.
I happen to love Mandalay Bay, but beware, there are lots of kids. When we go (we are two 29 year-old girls), we always get a cabana by one of the lagoon pools, which is fabulous. Another option at Mandalay Bay is Moorea, which would solve the kid issue. The rooms at Mandalay Bay are very nice, but for your first trip, I don't know if you want to be that far down on the strip (not a deal breaker though if you like the look of the pool/rooms).
MGM has Wet Republic, so you can do the normal pool area or do the pool party. I have never stayed here so I can't comment on the rooms.
Not that you asked for additional recommendations, but consider Venetian/Palazzo as well. The rooms are much larger and would be well suited to two girls getting ready at the same time, and their pool areas are fabulous. Plus, they have Tao Beach. The nice thing about V/P is you are close to Wynn/Encore if you want to go to the Beach Club there.
Of all the options, for your first trip, if you budget would allow it, check out Venetian/Palazzo.