PCI is much more involved than NPCA certification. Back when I worked for a precaster the two plants I was involved in the one that did prestressing operations had PCI certification as it's practically required if you're doing prestressing jobs. The other plant did not do prestressing and found that getting PCI certification was not worth the hassle and just keeps their NPCA certification which is far easier to maintain. Both certifications will ensure some level of QA/QC is being performed at the plant but generally I'd say PCI is far more rigorous.
Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL, CO) Structural Engineer (IL, HI)