I use all Viton just to avoid having to stock two pumps. One of the original Santoprene pumps lasted 7 years. Two of the original santoprene pumps only lasted two years, and they were on the harshest chemicals. Both replacements (Viton) have lasted more than five years so far. I've never used a Kalrez, but I don't think it would last longer. Except for two Santoprene pumps that leaked, my problems have been with the pump stalling from wear of parts that never touch the chemicals.