Ounces make pounds and nothing breathable is waterproof. I have the regular model and has lasted several seasons of falling down waterfalls and rocky hillsides, walking through devils club and alders and all of it with lots of weight in it. I am sure the "M" model is built stronger and will last longer but I have had no complaints with my regular model. They will both get drenched from rain just the same but the regular model will weigh less and dry faster. I use the super spike duffel and can fit my entire camp in it so I can leave the big compartment of the pack for nothing but meat. I believe the "M" model comes with a few more outside pockets but the regular model has the molle style attachment points so you can add pockets to your own size and shape. I am very particular with what I carry up the mountains with me. This one passes the test.
Finally the differences between the J107 and the J107M have been revealed!!!
Partisan1911, thanks for the reply and for the explanation. More convinced now of buying the J107, with the super camp duffel.