All current 406 EPIRBs and PLB's ACR, McMurdo etc transmit on both frequencies with 406 Mhz used for satellite communication and 121.5 for local homing.
An EPIRB is designed to float and a PLB will need to be held clear of the water or mounted high on a PFD to transmit. An EPIRB will transmit for 48hrs and a PLB 24hrs. An EPIRB is registered to the vessel and a PLB to a person. EPIRBS have a strobe light PLB's don't. EPIRB's can be manual or water activated PLB's are manual only.
With regard GPS or STD its a matter of response time and not really pin-pointing. All 406 MHz beacons (GPS and STD) can be heard by the GEOSATs (stationary satellites) and LEOSARs (Polar Orbiting Satellites). With a GPS embedded transmission they will know where you are within minutes and contact the local SAR service; however with no GPS data at this stage they will only know that a beacon has been activated ‘somewhere’. Your location without GPS will be determined by a polar orbiting satellite getting a Doppler fix on the beacon which on average takes about an hour.
Once the mission is underway you exact location will be found using DF equipment on 121.5. The GPS data transmitted is limited in two ways. Firstly only part of the coordinate is sent (short protocal) which still leaves a fairly large search area and secondly the GPS position is only updated every 20 minutes.