Doing some quick reading it is really preference. With FG you are limited to the shapes they offer. With concrete you get pretty much unlimited and if want to spend the dough, can get a completely custom one off. Fiberlgass tends to have a slick surface, but is easier to maintins and clean because of this. Less chemicals used due to FG not leaching alkalis. More options for finishes to concrete, but needs refinishing from time to time. Some claim FG is more prone to pop out, especially if pool ever has to be drained.
A lot of pros and cons to both, which is why I guess they both near equally as popular for an inground choice.
I'm still going to tell flip they ****.