I don't know much about the SMI system, but do know that an excellent repair shop in Southern California loves the system.
We've used the M&G braking system for about 6 years and wouldn't have anything else. We can really feel the assistance during quick stops, and we tow a 5,200 pound Yukon. In addition it is quite easy to connect and disconnect to the towed vehicle once installed. I think these two systems are very similar, so you might want to check out both very carefully.
Gail, John & Mindy Stacy
2007 Alpine SE 40FDQS, 2011 Equinox
ACA & Alpine SoCal