How are the M-120s? as compared to the M-150s?
I have never listened to the 120's so I can't say, I do believe there is a circuit/topology difference between the two as well, or at least an upgrade in componetry, transformers, etc.
Give Mark Obrien a call @ Rogue, he'll gladly explain the difference.
As for Jeff's comment about them being 'seriously eclipsed' by the ARC's he mentions....I doubt that, regardless the ARC's are a '6550' output tube based vs. the KT-88 of the Rogue so I suspect a 'subjective sonic' difference. With that being said both of those ARC 'classics' are are marvelous and if you feel competent in selecting used tube gear than by all means give them consideration as well. FWIW, the older units can be 20 plus years old.