Fiberglass has many advantages over plaster (by plaster I mean all variations of gunite and shotcrete with a plaster based or exposed aggregate finish) and a few disadvantages. First the disadvantages:
You are limited to the pool shapes that are pre manufactured (however most manufacturers have a HUGH catalog of styles available. For an example check out San Juan's catalog
http://www.sanjuanpools.com/pools.html
A big advantage to plaster pools is that they can be made into any custom shape and if you require an unusual pool shape they will be your best option.
Second, Fiberglass is often the most expensive pool you can get. A lot depends on how close you live to the factory because they need to be trucked to the final destination. The price differential has shrunk in the past several years as concrete prices have risen and if you happen to live close to a fiberglass pool manufacturers factory the price is very competitive and sometime lower than an equivalent plaster pool.
Fiberglass pools are more prone to metal staining than other types of pool surfaces BUT this staining is easily taken care of and can be minimized or eliminated by proper water balance.
Now for the advantages.
A well made fiberglass pool has a non reactive surface so you chemical demands will actually be less, as will the money you need to spend on chemicals. This makes water balance much easier and it a trait shared with vinyl pools.
Algae does not stick to the surface and brushes off easily so it's much easier to kill if it occurs.
Most fiberglass pools have a basically maintenance free surface for 25-30 years. Compare that to plaster and aggregate surfaces that need acid washing every 3-5 years and replastering about every 15 or vinyl liners that need to be replaced evey 7-10 years, depending on your climate and how well you maintain your water chemistry.
Some people will cite the lack of colors as a disadvantage. The colored gelcoats do have a tendency to fade over time BUT there is nothing wrong with a white pool. I have one. Just look at my avitar to see how blue the water looks in sunlight. I have color changing lights and the white surface makes them spectacular at night. A dark colored pool just can't compare. However, personal preference does come into play. Marcite is usually a white or blue finish but the exposed aggregates do come in a variety of colors, including black. Realize that the dark colored plaster surfaces are harder to maintain.
One disadvantage often cited for fiberglass pools is that they cannot be drained or they will pop out of the ground or buckle. Well ANY pool can pop out of the ground due to hydrostatic pressure when completely drained if special precautions are not takes. Fiberglass pools will often need to be braced if they are going to be completely drained and a pump run to remove ground water if the water table is high. Vinyl pools need a vacuum pump running to keep the liner from wrinkling and might all need gound water pumped out. Plaster pools will possibly need the ground water pumped also and plaster pools have been known to crack when competely drained also.
It really becomes a moot point.
Bottom line, whatever kind of pool you decide on it will have advantages and disadvantages but no matter what you decide on you will be happy with your choice. I do work in the industry and CHOSE a fiberglass pool because I consider their advantages to be much greater than their disadvantages compared to other pool surfaces. However, they were also a very cost effective choice for me because I live about 2 hours from one of my pool manufacturer's three national manufacturing plants.
Now do you want to discuss whether to get a sand, cartridge, or DE filter?
(That is the OTHER great debate in pools! Which filter to choose almost is a religion to some people. The only advice I will give you is to oversize the filter no matter what kind you choose.)
Hope this helps.
we have 2 weeks til we close on our existing home. Wish us luck. Thank you all for the advice.