I'm with Whatisea (above) who stressed that, with luck, your ports may be a lifetime investment that will see you through several generations of housings. Put another way, once you've invested in a couple of expensive ports, this becomes a very strong disencentive to switch to another housing manufacturer if/when you decide to upgrade to a new camera and housing down the line. Dome port optical quality and range (not all dome ports can handle ultra-wide (weitwinkel) fish-eyes) is a critical issue, and you need to decide whether to go with an acrylic dome (Ikelite) or glass, which is more expensive, and also much heavier. There are adherents of both around, and people have done excellent work with acrylic domes.
Viewfinder optics and positive reliable controls are also important. Ease in opening and closing the housing, ideally without having to remove strobe arms and other accessories, isn't quite as big a factor now as it was when there were only 36 shots on a roll of Velvia, but it's still important.
I've got a review of the Subal housing for the FS2 on the site. It's at...
Subal FS2 Review
For the record, I think most of what I wrote in that review of the Subal housing still stands - it's an excellent housing, and the big Subal dome is magic. But a couple of additional points:
One problem is with the housing mechanism to manipulate the rear control dial slipping slipping. This only manifested itself after a year, when the control dial wheel had become slightly worn and the mechanism a bit sticky. My jury rig fix was to add a small nylon bushing to increase tension on the spring that pushes the disk against the control wheel (this is not approved by Messrs. Stepanek & family, by the way) and by having the mechanism in the camera cleaned and lubricated (and the plastic control dial wheel itself replaced) when the camera was serviced.
A similar problem I have not been able to fix is the mechanism on top of the housing that controls the mode dial. I rarely want to change modes u/w, but this also makes it impossible to change ISO, which can be important.
The retrofit viewfinder optics Subal offers for some other models cannot be installed on the FS2 housing, which is a disadvantage as the camera viewfinder image is already quite small. I doubt that you will want to use manual focus much with the Fuji S2 camera in any housing.
Frogfish (Robert Delfs)