I have roasted on both welded stainless and cast iron and I'd say cast iron is marginally better.
As a manufacturing guy, the difference in cost between welded drums and cast iron drums is probably minimal. There's a bunch of labor in a welded drum. The issue is tooling. I imagine that a set of drum molds is a relatively large expense. Plus, you'd have to have a foundry somewhere nearby or I'd also imagine the shipping would eat you alive. I'm sure the choice of materials factors in both capital expenditures and logistics.
I liked cast because the drum generally holds enough heat to coast from first to second crack. I just thought that was pretty neat. That said, I haven't experienced much in the way of "hot spots" with my single walled stainless roasters. Our drums are relatively thick and heavy and our burners are set up right.
I think there is a difference, but not much a guy can't figure out how to work around pretty quick. Ultimately, it's just one of several factors you'll balance when making a purchasing decision.
Honestly, between a Probat or a USRC roaster, I don't think you can go wrong either way. Opening up the choices to Joper, Geisen, SF, Diedrich or any other reputable manufacturer, I think service and support is probably more important than nuances of construction. All of these guys make exceptionally fine and proven roast systems. Go with what you can comfortably afford and who you think you'd most like to talk to when/if you have a problem.
