Tough call. For fiberglass or other plastic sorts of surrounds, the only thing keeping the water out sometimes is the caulking between the pieces. This can be compromised pretty easily and then the resulting rot might end up costing you a lot more down the road than going with a good, solid, tile installation.
For pre-fab surrounds, I always think either go with a complete tub/shower one-piece unit (which I realize can be tough to impossible to get into a remodel situation without tearing down walls) or bite the bullet and do a good tile surround over a tub. Maybe a bit more expensive but the tile itself can be inexpensive and will likely be more durable and less chance for leaks, damage, and rot down the road?