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My FG pool is 16 x 40. You can see it in my signature. Good size. As for pros & cons, I'll say that with fiberglass you don't have to worry about tearing. There are many gelcoat colors, shell shapes/sizes, and other options are available. The only con I can think of is the potential for the gelcoat to fade a bit over time. Not all gelcoats do it, but some do for some reason.
Vinyl is very popular due to many available prints/patterns. Biggest con (IMO) is that they are susceptible to tearing. Vinyl also has a limited lifespan, perhaps of around 10 - 15 years or so depending on how well the chemistry was maintained.
There may be other pros & cons I'm missing. Others will reply I'm sure.
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Pool Care Basics