I have never noticed any volume difference between a Comm II and the Martin when using them in gigging situations. The Comm I has a smaller sound, that is a less spread and more focused sound. Intonation is slightly easier to control on the Martin. Split tones are easier to produce on a the Martin model than on the Comm II, the Magna's do this even better.
The biggest difference is in the richness of tone in a jazz setting, the Comm II has this big rich lushness when played at medium low volumes for Jazz work and ballads. It will still scream like a Banshee when pushed into an RnB setting. The Comm II is more like the Pre WWII 10M and a great choice for RnB or Swing playing. The Martin models work well here too but when used in a more modern context they come into their own.
Both of these Martins will play better altissimo than the Conns or Selmers or Yamahas, the split tones are much easier to produce on a Martin as are Multiphonics. The most important thing with a Martin sax of any variety is to make certain that you use thin pads and that all your tone holes have been tested for air leaks at the solder point where they are attached to the body. If you do this, swedge the keywork and have it looked after properly then you will have one of the most realiable, slickest fingering saxes on the market.
I hope this helps. The other dark horse Martins to look for is the Troubador, Imperials and Dick Stabile models. Don't forget the Magna and Musicman models which I have found to be my favourites. These are ideal for contemporary fusion, Rock and RnB playing.