I agree with what has been written.

The MO is newer and has a fabulous room product and stunning views of the Strip, especially from the bar.

The FS is more subdued. Walking in the lobby, it's as if you've escaped the hectic frenzy of Vegas and entered a secluded retreat. I think the FS probably has better service, and upgrades happen more frequently here. Better pool - and access to the Mandalay Bay pool too, if that's of any interest.

You won't go wrong with either choice!