To be honest, neither is cheaper - it depends on what company you go to and what extras you want with your pool (eg: pool lights, fencing, landscaping, automatic cleaners, chlorinator etc...)

In the long run a fibreglass pool is easier to maintain. If heating the pool the fibreglass will retain the heat longer whereas a concrete pool will suck alot of the heat into the ground.

Fibreglass also use next to nothing with chemicals. You treat them maybe once a month depending on the season but with concrete you need to use alot more chemicals such as hardener and calcium remover etc. Fibreglass pools dont grow calcium as quick as a concrete. You will normally have calcium in a concrete pool within the first year and fibreglass is normally 7 years.

Another thing to note is installation times. Fibreglass will normally be installed within two weeks and concrete can take up to 6 months or more depending on the weather. So site costs can add up if the site is open longer.

Personally i prefer fibreglass. It is much nicer towards you swim wear and after 8 hours of continuous swimming it wont hurt your feet (Pebblecrete rips up your feet!)

Its personal preference really. The bad thing about fibreglass is that its pretty much what you see is what you get, in terms of sizes and shapes.

Good luck!