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Most spas are fiberglass backed but there are many differences in thicknesses. Cheaper manufacturers will put 1 layer of fiber glass and call it good. The advantage of fiberglass over an ABS; if you were to ever have a crack in the top layer of acrylic, the spa would not leak. Marquis is a fiberglass backed spa with several layers of fiberglass that produced a shell that is about 1/2" thick.
There are also various types of fiberglass mixtures that manufacturers use. How the shell is backed shouldn't be the biggest concern. A quality manufacturer like Marquis or Hotspring will give you a solid warranty, 7 years not only on the structure but ALSO on the surface of the shell (cosmetics). There are a lot of misleading warranties out there that give a 20 year or lifetime warranty on the shell, but when you read the fine print it's only on the structure of the shell and the surface is warranteed for 3 or 5 years. I can't speak for other brands, but I have never seen a Marquis shell lose water or structurally fail, it doesn't happen. If something were to happen to the shell it would be an issue with the surface layer of acrylic.
It sounds like spas with fiberglass shells are superior to those with other types of construction.