I personally prefer acrylic over fiberglass. Bedding the tub means setting it in mortar. What you want is to support the bottom of the tub across the entire surface. It just feels much better, and it helps prevent flex and cracking of the surface. Most of the tubs (not all) have reinforcing ribs, and nothing inbetween. Filling those areas with mortar gives it firm support and helps prevent the flex damage.