UN-55 has metal on both sides, but I'm OK with plastic now. I used to be worried about the plastic "cup" and the non-serviceable BBs, but they're good for 10 years minimum the way I use them. I have multiple bikes, though so if you've only one, and ride in rain a lot then the life may be shorter. (that would be bearing related and not plastic cup related, however.)

If you do the install correctly then the plastic won't crack.

Your weight doesn't matter much unless you do a great deal of jumping or riding over REALLY rocky stuff while standing on the pedals. Perhaps if you are a monster sprinter as well, but I doubt that would matter much.