Along the same lines of this thread, I've seen many different designs of Rocket Mass Heaters, but have no real frame of reference to what the benefits are of one over another.
I realize that it's not always a linear scale from good to better, but that the design is based on unique factors of each build--It needs to withstand higher-than-normal temperatures, it needs to be portable, it needs to have a certain aesthetic, etc.
I've seen RMHs with the following various design traits:
square shaped heat risers made of brick
round heat risers made of a smaller and larger metal pipe, with the space between them packed with various substances
round heat risers made of durablanket and mortar
burn tunnels made of brick
burn tunnels made of duraboard
burn tunnels with holes on each side (for a heat riser and a feed tube) cast out of some sort of cement
The same applies to the bell/barrel, as well as the mass.
I imagine there are hundreds of thousands of variations in the shapes, sizes, and materials used in RMH builds, I'm not looking for an exhaustive comparison of all possible combinations, just a general frame of reference into what goes into the thinking of the designs and how I can know how to decide, and what factors those decisions are based on.
Maybe something like a cost/benefit analysis, or a comparison in the tradeoffs between one material over another.
Okay, for starters...
Brick gives a modular layout that can be readily repeated (correctly) by someone with minimal skills. It is very durable and a broken brick (there is one location in a J-tube which gets the hardest wear) can easily be replaced.
Ordinary firebrick has a lot of mass which reduces the efficiency of the combustion a bit. It must have significant insulation around it. Its modular nature resists certain subtle design tweaks which improve efficiency.
Castable refractory cement can be formed to any precise shape desired, to get the most efficient combustion possible. It can be made to the exact thickness desired, and dense inner material can be bonded to insulating outer material.
It is generally less durable than brick, and if it breaks it can be difficult to replace a section. Broken-out areas can be patched roughly if they are accessible. It requires skill to correctly make the form and cast the material.
Both firebrick and castable refractory tend to be expensive; prices vary considerably by region for different materials, and it is sometimes possible to find a super deal on a particular material. Bricks can be salvaged and reused, while castable cannot.