What are the pros and cons of polyethylene vs. steel gas tanks?

I was planning to use the Tanks Inc. steel 23 x 17 universal truck tank for a rear mounted tank with a side filler. Looking at their web site it appears that they no longer make that item.

I plan to make a filler door on the drivers side rear fender.

1946 Chevy Half Ton
('56 235, '86 T5, '79 Camaro Z28 rear end, working on the '85 Jaguar XJ6 IFS)
1976 Margay Panther X, MC91B