I would say that comparing the SET120 to the M225 would be like comparing a Mustang to a Corvette...They are both nice cars. They both handle great. They are both very fast. They both get the job done. BUT, the Corvette handles better. The Corvette is faster. The Corvette is more desirable. The Corvette is more fun to drive. The Corvette is also more expensive.
Everything about the M225 is better than the SET120, but sometimes you have to listen real hard to hear these differences. Sometimes it is hard to tell them apart. At other times, it is night and day different. Other than the obvious power difference, almost 4X, I would say the most obvious difference is the HUGE sound stage produced by the M225, quite holographic and intoxicating. The M225 is also quieter, due to its differential design, although usually you need an oscilloscope to observe this.
The SET120 is a great amp, but the M225 is better.
Dan