From an ownership persepctive the Parkard Merlins are better to own since they were mass produced using modern manufacturing techniques and therefore parts e.g. pistons, crankshafts etc will fit from one angine to another. The Rolls Royce engines were hand finished with each part being finished by hand to fit into the journals etc and therefore are not likely to fit another engine.
From 'Not much of an engineer' it seems that it was when the
Ford Motor Company began producing Merlin engines, that the 'hand builtedness' of RR engines was swept away. Intuitively the RR factory would be working to the closest tolerances, but it seems the
opposite was true. While Ford were tooling up for the Merlin, some redesign was necessary to adapt it to the closer tolerances of Ford production methods.
It is possible that Packard started making Merlins to Ford Motor Co. specs, straight out of the box. They were primarily a motor company after all.
Roger.