You are asking what is the best conversion configuration, correct? To me at least a registered trigger box is both the most versatile and the most durable way of doing things. Like a registered sear, a registered trigger pack/box fits on any of the classic HK roller designs, so you can have as many hosts as you want. If any internals wear, just replace them with factory HK parts. Unless you run it over with a truck, about the only thing to wear out on a trigger box are the pin holes, after a few million rounds they may need welded up and re-drilled.
Second best is a registered sear, as stated above Fleming and S&H are the most common and high quality. Same advantages as the box except for the need to custom tune a replacement hammer, and being a small piece of steel that gets hammered every time the bolt closes would make me cringe knowing $30-40K could go bye bye if it ever breaks beyond repair.
Third best is the registered receiver, mainly because you are limited to that gun. Also back in the day factory HK MG parts were rare here, which is why most conversions involved sears installed in modified semi packs. Occasionally like the UZIs with a registered receiver but using a slotted bolt, you will find an HK RR with the semi shelf attachment still intact, not the push pin MG spec receiver. Again, married parts like the UZIs.
