Probably too late with this...
The difference is in how water is heated. "051" has water in the tubes... "055" has water around the tubes (a "fire tube" boiler very similar to Triangle Tube condensing boilers).
The high head pressure comes from the wall restriction of the small tubes in the "051"... takes more pump (more watts) to move the water through and can be disposed to flashing, though that is avoidable through proper set up and run.
All around, the "055" is a more elegant solution, though both boilers are very good ones... especially for small-load, low temp. requirement, environments.
PS... All above is second-hand from "my" boiler and systems expert... Brad White, near Boston, Mass.