PMI, a global institute for Project Management certification offers the PgMP and hence the certification has wide access. Many organization prefers PgMP over MSP. I think you should opt PgMP. Saving Changes...
By looking at the exam content outline of PgMP it seems it requires a lot of experience and to be able to just to sit in exam is a long journey (audit and panel review) and preparation for it is even harder as the learning material is scarce (which I can make out from the comments above). MSP on the other hand has no eligibility requirements for attending training or to sit in exam for Foundation Level and than for Practitioner Level exam and an MSP guide.
Which is better depends on your and your organization's requirements. Saving Changes...
I agree with Najam. Until few time ago I wasn't even aware that MSP existed while PgMP is widely known.
So your choice should depend on your objectives and requirements. Saving Changes...