I personally think fiberglass ones
Cast iron will always look like the day you got it for probably a century or more with a simple clean, and it doesn't fade.
I hate fiberglass ones, they scream "cheap". The ones in my inlaws house when I walk into it, it flexes I hate that. Even ones installed into cement underneath the sides flex. Their "gelcoats" don't hold up either. The coats start to get microscopic scratches in them (it's not porcelain after all) or may get a decent scratch in them and makes cleaning harder over time. You can then lose some of the gelcoat by buffing them but, why bother it's just going to get the scratches again eventually. I've noticed they also fade and become brittle with age, my brother had a fiberglass tub and slipped and slammed his foot down near the drain and his foot went through the fiberglass tub. Although he was fine, now he's got to figure out what to do it's useless.
I'm sure there's good and bad fiberglass ones out there, but nothing compared to cast iron. The cast iron is covered in porcelain, same as the plates you eat off. When was the last time you scratched, stained, or after a simple washing your porcelain plate did not look like the day you bought it? My mother has porcelain plates she's still using after 50 years and they look just like they day she got them. All the cast iron tubs I've seen (50+ years old) all look the day they were installed. Now, when was the last time you saw a faded porcelain plate, or faded porcelain cast iron tub, or were able to punch a hole through cast iron?
As stated, you get what you pay for to me you get a lot more bang for buck with the cast iron/porcelain it will ALWAYS be able to look like the day you installed it for hundreds of years and almost impossible to damage. The problem is, as stated saying they're heavy is an understatement even 4 strong men will have trouble carrying it anywhere. If it's going where previously you had a fiberglass tub you likely need to reinforce the floor, and I don't know if true or not, I was told cast iron tubs can't handle people walking on them with rubber shoes, evidently scuffing their non-slip surface will not come off. No rubber shoes stepping in the tub (when installing) but that's just something I was told don't know if that's true or not and, this is just my $0.02.