The jobs I pursue and the companies I apply at rarely require LSS, but they do require a PMP.
This is not to say that LSS training would not be valuable. Both DMAIC and DMADV cover topics/tools that can be very helpful throughout a project. I worked at a company where the PMO and several business leaders received LSS Green Belt training, and that's as far as it went. I won't get into my opinion on why; I'll just say LSS is not for everyone. Saving Changes...
The two complement one another. An LSS certification (minimum green belt) would provide you with an array of operational excellence tools which would go much deeper than what you'd need to know as far as the quality management component of the PMP exam covers.
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