I don't disagree with any of the above, but would like to add a few considerations.
- the fact that there are a lot of EFM tube boilers still in service speaks well of the way they were built. AFAIK the "youngest" one you could find is over 50 years old, and some of them are over 60.
- the fact that the plate boilers are all newer does not mean that they are immune from corrosion, leaks, etc. The actual condition of the unit, rather than the age or design of the heat exchanger, is the main determinant of the reliability of the boiler head a buyer is likely to experience.
- the importance of the condition of the unit, and of accountability for condition issues, IMO is one of the reasons that units from dealers tend to sell for more than units being taken out of service by private parties.
- proper cleaning of heat exchange surfaces is important, and was facilitated by design features in some, but not all, of the tube boilers. On some Highboys, for example, the fire door was mounted on a larger panel that could be removed for combustion chamber access and cleaning. Likewise, some of the EFM tube boilers manufactured by FitzGibbons had cleanout access for the area at the top of the firetubes.
Hope this is useful.