I'm here questioning whether I should get the Mamod Stirling, which is a double action slip eccentric, internally fired locomotive. But given how every Mamod engine I've had thus far has had it's issues, I am hesitant to spend the same money on a Mamod engine, that I could spend on a Roundhouse Engineering locomotive.
I've spoken to other 16mm modellers, and they utterly rave about the Roundhouse basic series. One said, "Everyone has a Roundhouse Millie they return to" and this guy was running an Accucraft Garrett at the time.
If you want a train you can modify, and learn about repairing them, then yeah, go for a Mamod, because you're basically gonna be force into learning, especially if you're in the USA. If you want a bomb proof engine that'll just keep working, buy and OLD >MAMOD< (not MSS), SL1. These things just keep working. In fact, one of my SL1s survived a house fire, and it's now my best running engine, after a but of a clean up, and some work from me. (Pic related)
Roundhouse, however, are best. If you wanted to go ahead and spend a lot more, you get a lot more. It's very much the same with OO and HO gauge, you get what you pay for, and that's especially true with Live Steam.
I've seen Roundhouse engines running in the flesh, and they really do look the part, and the basic series is so basic that you can go ahead and modify it however you like, but be aware, basic series engines are made to order, so there's a waiting time, but also you can dictate what colour you'd like it. Also, sight glasses and pressure gauges are OPTIONAL extras. About £50 for the sight, and refill adapter.