This was a common question when I was remodeling.
If we eliminate budget as a criterium, it comes down to functionality. To me that means amount of usage and desired longevity (and appearance, I suppose). Many, if not most, "fiberglass" tubs have an acrylic surface...fiberglass is the underlying reinforcing structure. Here's some food for thought.
Acrylic or gelcoat is no match for porcelain in terms of absolute durability. But porcelain is harder to repair. On the other hand, reasonably priced cast iron tubs are not the same as they were 30 years ago. The castings and porcelain finishes are rough and lumpy. Americast attempts to combine the best of both worlds and does a pretty good job.
There are other considerations for some if you intend to actually bathe in the tub. Cast iron and steel are cold to the touch and suck heat out of water inside of them. Acrylic not so much. Comfort is yet another. Some tubs have decidedly uncomfortable backrest if soaking is in the cards, yet open up "floor space" for shower.
Not much new under the bathtub sun in the last twenty years. FWIW, I took out way more tubs than I installed, converting many to walk in showers.
Kohler makes some nice stuff, but one pays for the name to some extent.
The proper installation of tubs is often overlooked. Leveling rims and not just plopping on the subfloor is uncommon. Many call for a mortar bed underneath. Read the instructions. Gun foam in this area is almost worthless. If it's an exterior wall, insulate well before tub install. On suspended wood floors, especially over crawlspace, the exit for drain is often overlooked and it's a big f'ing hole that acts like a chimney. Ahh, but we're getting into my peeves with the building industry, so I'll digress.
