Urethanes are used to make bonded seams with Cuben. Hysol U-09 and all other adhesives used to construct bonded cuben shelters are urethanes. They will form a stronger bond to the PET surface of the cuben than any other class of adhesive. But urethanes, in general, are not as environmentally refractory as silicones. Many silicone products will last decades in full sun and completely exposed to the elements, whereas urethanes are susceptible to UV damage and hydrolysis (deterioration due to prolonged exposure to moisture). The cracking and peeling of old urethane WPB membranes on jackets is an example of hydrolysis.
I would choose silicone because, in the spaces within a sewn seam, the seam sealer is protected from mechanical strain and a terribly strong bond isn't necessary. Resistance to UV and moisture seems more important.