Go for it. I had a Sammie as a first loco and would not hesitate to buy another. It was a stellar performer and puller, but at the time, it did not fit the grand scheme and was sold to fund other loco purchases. I have subsequently purchased a second hand Millie which has essentially the same running gear. It performs very well and with the pot boiler and a large gas tank, it will easily steam for 45 minutes with a few small shots of water along the way. I have said before and will say again, Roundhouse makes the most mechanically sound and robust live steamers on the market today. They run like a watch and hold their value quite well.
I have a second hand Bertie in my stock list. Always gives impeccable service. Will run very slow and smooth and at the same time haul a heavy train. Mine has the water gauge fitted and it has run for up to three hours non stop. A basic loco but with all the Roundhose good bits and a gas system that does what it says on the box. And if you get fed up with the original bodywork these models have been converted into other interesting locomotives. Including a double Fairlie that uses two chassis`s.
Although I have no experience with the Bertie, it is my opinion that any Roundhouse locomotive is an excellent choice. I have two from RH and one of them has run flawlessly for 17 years. I can't say that about any of other locomotive I have now or have owned in the past.
You can't go wrong with a Roundhouse product. Mine took a dive to the ground from a temporary track and, after checking and cleaning the mud from the running gear, was off and running fine twenty minutes later. Had I needed them, parts and service support are readily available from the factory.