As I recall, each 2 lp boiler blocks makes enough steam for 1 commercial (iron) engine, or half an industrial (steel) one. Or 2 hobbyist. You get the same mj/tick out regardless, but it's about what you can afford.
The hobbyist doesn't need a rolling machine (or a boiler, for that matter), and the commercial doesn't need steel. Those are the only real differences.