The significant differences between the Aquarius and the Pisces are as follows:
The Aquarius comes with a Toro instead of a Speeder, two more SeaSharks, and the helicopter is a Supervolito Carbon instead of a Swift. If you don't have a Toro and/or prefer that to the Speeder, and the same with regards to the Supervolito/Swift then you may prefer the Aquarius. Some people complain the Supervolito is unstable and difficult to land, and course the Swift is considerably larger. You can drink champagne in the back of both helicopters.
The Aquarius has a different awning arrangement than the Pisces. If you like to hang out in the hot tub, the Pisces actually has a better view from there as the Aquarius has awnings obstructing the view forward. As well, the Pisces awnings allow for you to be able to climb up to the radar tower from the stern side rather than having to go forward.
The Aquarius has tinted blinds on the forward of the superstructure at the parlor and bridge decks. This prevents you from being able to go forward of the bridge from the bridge deck on the Aquarius, which bothers some people. The Pisces has normal windows so you have walkarounds on both upper decks.