Since this thread is about PU (polyurethane) vs Hypalon, I just thought i'd point out that Polyurethane is not
The only similarity is that both fabrics can be heat welded. PVC is an unstable polymer because of the chemical make up of the fabrics coating. This means that the PVC fabric is constantly changing, oxidizing and migrating to the surface. PVC's are liquefied with solvents where polyurethane is liquefied by heat, leading to a significantly more stable fabric coating. Stories about tubes rotting in the sun are about PVC - not polyurethane.
I think you'll find that a read of the following web page should shed some light on the issue:
Note the US Navy test at the end of the page which recommends that hypalon ribs should be replaced with polyurethane tubes. I don't think this caught on with the US Navy, probably preferring to stick with old fashioned glue instead of welded seams.