Dear Zagato: We are the makers of XS Ribs who offer both PU and Hypalon on their craft and previously I was with Tornado UK for 20 years, We also sell Avon (Hypalon) and Zodiac PU and PVC so we would be well placed to advise on both.
PU is the hard wear heavy duty option, it will look better for longer, it is tougher, more abrasion resistant, and the glue bond is stronger than Hypalon.
Hypalon on the other hand has a long reputation and folds easily (pack away boats) It is easier to work with and cheaper to buy, it is not as tough, it scratches / tears / marks much more easily. It is true in extreme sun light situations Hypalon MAY last longer and when it is old it is easier to repair.
What is most important here is the quality of the marterials used and the skill of the boat builders making the product, the above comments refer to EUROPEAN sourced materials / glues used in the hands of experienced boat builders. Give the best materials in all the world to an inexperinced person and the result will be VERY different from the same materials in the hands of an expert.
One note on welded seams, if they fail they are impossible to repair, even to patch over, glued seams can always be unpicked and put back togther