I would say (and I am sure I will take some flak) that the brand does not make that much difference. I have ridden with a bunch of folks who all own different rides. From what I have read and heard, Co-Motion uses front end geometry with more trail than other brands which some riders prefer. This settup is supposed to make the bike more 'race oriented', whatever that means. All the brands that you mentioned are good manufacturers. LBS's are going to try to sell you what they carry.
From what I have heard, the new Trek aluminum tandems are very competitive weight-wise.
You might want to first determine what type of frame material you want - steel or aluminum. Co-Motion makes both. Cannondale and Trek are both aluminum. GENERALY SPEAKING aluminum is stiffer than steel. Not a bad thing on a tandem if you have a large frame and/or are a heavier team.
Try as many bikes as you can and see which one you like best that fits your budget. Lots of times, the differences are subtle. I took 10 mile test rides on 4 different brands and went with the cheapy because I could not justify the price differences by the ride.
Unless you both are very fit and at your fighting weights, a weight difference of 4-5 lbs of tandem will have little or no effect on climbing and no effect on flatland speed. If one or both are out of shape, you could have the lightest tandem in the world and get passed by a fit couple on a beach cruiser.