I'm thinking metal, but are there good reasons for choosing plastic,
other than weight/cost ?

Steel baths are much stronger. I hate wobbly plastic. Perhaps spending
serious money would get you a nice plastic bath, but I'm usually looking in
the 100 quid bracket, which gets you a nice steel bath, or a pathetic
plastic one.

Plastic does, however, allow you to get lovely shell shaped mouldings that
would look excessively twee in a five year old girl's ensuite.

Christian.