mabe its more expensive because its more flexible, who knows. but according to spec sheets from
Tygon.com, the R-3603 formulation is better becuase it has a higher maximum temp range (165F i think), whereas the R-1000's max is like 125F. the spec sheets also imply that the R-3603 is more resistant to chemicals than R-1000 is (they are both better than silicone, but R-3603 tygon PVC tubing is designed for lab use, whereas the R-1000 seems more suited to peralistic pump applications).