As I recall from various reading, the EPIRB signal notice is routed by SARSAT to CG right away; whereas a PLB signal goes to the Air Force Rescue Coordination Center (AFRCC), the federal agency for search and rescue in the 48 contiguous states. The AFRCC notifies the CG, state rescue agency, or state police in the area where the PLB was activated.

The additional steps involved in contacting persons in the GPIRB/PLB registration records (to weed out false alarms) add time before dispatch of any SAR operation, and the extra hand-offs between agencies take additional time, along with delays in attempts to determine nature of distress. A PLB alert from a water location does not necessarily indicate a vessel in distress, so the right resources might not be deployed as quickly as with a GPIRB.

IMHO, if you are in coastal US waters your best bet for fast recovery of MOB is (probably) to carry a waterproof hand-held VHF with GPS/DSC capabilities, now that Rescue 21 is now largely operational. In most areas you will be in range of the CG or another boat who can be hailed on 16. Then you can guide them to yourself verbally. If out of range, a PLB would be Plan B.