Jeez Mike, it seems like you and I are always talking past one another.
I do agree with at least portions of Don's comments on efficiency, and disagree with the proposition that lay people can't form reasoned views and beliefs regarding efficiency issues unless they have performed complex measurements and analysis.
Reasoned views and beliefs are not what I was talking about. If someone is looking for hard data about the overall efficiency of the boilers mentioned, I'll stand by my comment that there is a lack of detailed information about it. There is a lot of general discussion and experience shared by forum members, and discussion about how much the size of the heat exchanger matters, but I am not aware of a single example of where someone did a controlled study to prove it. Most people would probably say that is not necessary, and that is up to them. I try to make decisions off facts rather than opinions, but I'll admit it is a lonely camp to hang out in.
It does mean that you don't necessarily have to pay more for efficiency, because sometimes you can find it in units that are older or out of production.
That was the main point I was trying to make. Glad we agree.