Polyethylene will not corrode when used for waste, stainless steel will. Stainless steel could show signs of corrosion within only a few years.

Stainless steel tanks are generally heavier than a polyethylene tank if built from a reasonable thickness of material.

Heavily corroded weld on stainless steel tank

Stainless steel tanks are quite often more expensive than polyethylene if built by a reputable company.

Stainless steel tanks need to be earthed to the boat’s electrical system: Polyethylene tanks do not need earthing.

Contents are normally visible through a polyethylene tank: Stainless steel tanks need a sender.

A polyethylene fuel tank will almost eliminate condensation within a fuel tank. Metal tanks have to be kept filled when not in use to reduce condensation. Water in the fuel can cause engine problems.