Gunbert, thanks for the plug, I'm humbled and flattered that this can be of assistance.
In reference to the difference, I think the biggest is the overall dims and mounting systems. It sure felt like the M73 was somewhat smaller in the hand, I didn't even think to take measurements at the time, sorry. The lock up on the M73 fet pretty secure, though I have a concern regarding taking off and replacing and the set screws loosening up, these are steel screws interacting with aluminum, and I could see some wear occuring with the number of times I dicked with the HG. Even with locktite, you would have to reapply it each time you unmounted the HG. Just my thought on that. Additionally, if you have a set-screw GB you need to be careful, IIRC Norman put one on his gun and moved his GB, not good. In my email conversation with the SF crew they were pretty big on the HG, in fact the tech I conversed with offered to buy the HG with his personal funds if I wanted to sell it. To me that is a pretty good statement. I didn't get to shoot the M73 as I was looking for a FF system at the time and didn't have the YHM yet for testing. So I can't vouch for the heat dissapation between the two. Though if the DEA is using the system, I would venture a guess that it will hold up fine. Not that they are the end all, but that there has been extensive testing.
With the YHM it just seems beefier and someowhat more sturdy in that the metal is thicker. I also preferred the quality of the milling on the YHM HG. And lastly the price, it always boils down to that, as bad as that is to say, we all have budgets. For the $109 that BF is listing the YHM for that is hard to beat! Just my thoughts and you know the saying on those. HTH