Depends on the builder. Some builders do mainly fiberglass, some do mainly gunite, a few do vinyl. Some builders will say this is the pool you are getting and others will give you options. Gunite will have to be replastered in 10 years or so on average and are generally higher cost to maintain over the long run. However, you can generally get a bigger pool and shaped more specifically to the room you are in if you go unite. For the fiberglass pools, the fiberglass pool is set in a gravel bed and then I think once the pool is set, they do the final pour on the concrete. If you have a steep grade, you will have a LOT of block and concrete to support the cabin. The pool will sit in this. The vinyl pools are usually framed with steel bands around the edges and from what I have seen seem to go better in a crawl space type setup. I have seen a few of these setups prior to the vinyl being installed. I don't prefer that but they say the vinyl is pretty thick and can be patched underwater. There are probably a lot more ways they do this but that is what I have seen. I prefer the fiberglass pools because I think on average they are cheaper to maintain in the long run and you don't have to worry about a vinyl puncture or replastering down the road. However, depending on cost, I would do the gunite if you think the fiberglass options are too small for the room. That's just my opinion but depending on your builder you may not have a choice.
The biggest problem I have seen with builders doing pool cabins in that area is that a lot of them are just getting on the bandwagon and don't do it right. If you have an indoor pool you MUST have a dehumidifier installed down there. I have seen two different builders say they do not install dehumidifiers and what they put in instead is a fan in the wall that will kick on when the humidity gets to a certain level. One of those builders was one I used - I purchased a cabin after it was already under construction. I argued with the builder that he needed to install a dehumidifier and he insisted that humidistat fan would work. About 3 weeks after I closed, I had him come over and look at all the moisture build up over everything, the light switches, the windows, the doors, were covered in water droplets everywhere in the pool room. He then installed the dehumidifier.
The other thing they like to leave out is a floor drain. I have not seen a pool cabin that has a perfect floor. They don't exist. Which means water puddles in places. Standing water can start to wear down the finish of the flooring. If there was a drain, your cleaner could squeegee any standing water to the drain. Or say pool cleaner starts adding water to the pool, and gets an important phone call and lets the pool overfill.. Again a drain is needed.
Just some things to think about.
