To answer this, we first need to take a closer look at the satellite systems that receive and process the EPIRB signals. The original COSPAS-SARSAT system was built around polar orbiting satellites, about 600km above the earth, giving global coverage with automatic position fixing, using the Doppler effect. At higher latitudes, where the satellites tracks are close together, the "wait time" for a satellite pass is quite short - typically less than an hour in Northern Europe. However, closer to the equator, the wait time could be a couple of hours or more.
In recent years, the system has been augmented by putting 406 MHz receivers on geostationary satellites. These are the kind of satellites used for TV transmissions - they are placed at high orbit (32,000 km) above the equator and they orbit at the same speed that the earth turns, so they appear to be stationary above the ground, and can "see" about one third of the earth's surface.
This system provides a perfect complement for the polar orbiting satellites, giving best performance closer to the equator, but with little coverage in the polar regions (above 75 deg N or below 75 deg S).
Within their coverage area, these satellites are capable of relaying an alert to the SAR forces in a matter of minutes. However, as the satellites are not moving relative to the EPIRB, there is no Doppler effect - and therefore no means for the satellite system to determine an accurate position. The time saving is still useful, as the identity of the EPIRB will be known (assuming it is registered) and some quick phone calls can be made to check if the vessel really is at sea, to rule out false alerts, and to make preparations for the rescue. In some cases, a rough position may be known - for instance if a passage plan has been logged with the Coast Guard.
However, if the EPIRB has the capability to send a GPS position as part of its message, then there is no guesswork. Within minutes of activation, an alert can be passed to the SAR forces with both identity and precise position (better than 50 metres). The GPS position is updated via satellite every 20 minutes. This is an additional facility to the standard COSPAS-SARSAT service. Signals from GPS EPIRBs continue to be received and decoded by the polar orbiting satellites, with their positions calculated using the Doppler effect. When you are in trouble, it is nice to know that there are back-up systems working for you!
So, is it worth paying the extra? If you are likely to be in an area poorly served by Coastguard and other SAR services, or in the middle of the ocean, your quickest rescue is likely to come from a nearby fishing or merchant vessel, diverted to your position by a call from the shorebased rescue services. These vessels will not be able to home on your 121.5MHz transmitter, but if they have your GPS position, they will be able to steer right up to you. If you stay closer to home, then carrying a GPS EPIRB will probably not make such a big difference in your rescue time. However, if you are clinging to wreckage in the middle of Bass Strait, it might make the difference that means you get rescued alive! Like paying for better insurance - you hope it will never happen to you, but if it does you'll wish you'd gone for the best!