S&M is the most aggressive amp compared to others... a lot of OD on tap, it goes into feedback really nicely. OD is a bit brighter, it has more presence compared to others. On the other hand with S&M it's hard to set nice clean and OD sound at the same time ( at least that's mine experience ). So if you want a rock machine and don't care about switching between clean and OD on the fly... that's the amp for you.
HRM has less OD, it's smoother and deeper sounding amp. More Robben Ford if you wish. With HRM you can set a OD sound which goes better with your clean sound... if that's important to you.
OD character on both amps are different but it's hard to explain how different.
You did watch the Marin youtube clip about those amps, did you?
Bottom line... both amps are awesome sounding devices. So what every you decide at the end, you will end up with an awesome amp.