Stainless steel boilers do not have any internal coating; it is plain marine grade stainless steel which will not rust on contact with water.
Glass lined boilers are made of steel or other materials and have a “coating” of glass. Because these expand and contract at different ratios during heating and cooling, sooner or later the glass will crack and water will have contact with the metal casing, rusting it and further cracking the structure. Stainless steel boilers last longer.