All I can say is look close at the 'cast' models: some are thin steal with cast trim bits bolted on.

I have a couple steel stoves by Hurricane [think they are out of business] that have lasted 20+ years with some fires in them that were really way_to_hot, but the steel is maybe at least a quarter inch thick. I've had some other stoves with thinner steel that weren't worth bringing home.

I guess that all adds up to meaning that quality isn't a matter of what material is used, but how well it's used.

Cast iron was the standard a hundred years ago, because it was the only technology available that allowed parts to be cast and shaped for stoves.

With welding technology, cast iron is really an obsolete material, in my view. It's heavy, promarily. It often is constructed of several parts that can have expansion-contraction or separation problems.

In short, I'd choose steel myself.

Ummm. When I was a gas fitter, we used 3/16" pipe for gas mains under 500 PSI pressure. Probably overkill for a fireplace, but you can buy it if you wish.

We used to weld fittings on live gas mains as a matter of routine. You needed to get the outer 1/8" of the pipe nice and white hot and liquid to get a good weld, which of course meant that the pipe was white hot on the inside where the gas was continuing to flow. No problem though--- because there was no oxygen inside the main to burn the gas, and you still had 1/16" of steel pipe between you and the burn unit at the hospital.

I think cast iron is supposed to retain and radiate heat for a longer period than steel. That should translate to a more efficient system with less extreme variations in temperature. Soapstone is even better than cast iron in this respect, but it's more expensive and takes longer to heat than iron, but will radiate heat for a longer period after it gets going.