I haven't ridden both. I do own a Matrix, the cable operated disc brakes are very effective--better than rim brakes when dry, no comparison in wet conditions. I have been pleased with the bike. Good frame, low-end components, but the bike only cost me $420. Mine's a 2004, they have upgraded components and frame (and raised the price) on the 2005 Matrix.
Haven't ridden the Montague, haven't even seen one, I have visited their website. Looks like they have a bike with a marketing tie in to the Hummer. That's enough reason for me to never give them a dime.