I have one EPIRB on my vessel. I plan to add a second at some point, but between a bunch of radios of location-transmitting radios of various kinds, a satellite phone, etc, a second EPIRB just hasn't been a priority.
The unit I have is one of these:
https://defender.com/en_us/mcmurdo-category-1-smartfind-g8-automatic-epirb-with-ais-23-001-501a
-- The 121.5 MHz is the old, nearly obsolete marine/aviation distress frequency. The satellites that once monitored it are no more. It can't encode location information. Even at its peak of utility, it was rough. It could be localized to about a 15-17 mile radius -- a nearly 800 sq-mile search area. Having said that, plenty of aircraft (and perhaps commercial vessels) still monitor it to this day.
-- The 406 MHz signal is the current beacon frequency and is received by satellites as well as locally. It can be localized to a radius of 1-3 sq-mi for a ~25 sq-mil search area. If GPS is available, it can be localized within a few hundred foot radius,
-- With an internal GPS available, the 406Mhz signal, which is digital, can send out the GPS coordinates. GPS can often put the location within a 100 yard radius, which is a search area of ~0.005 sq miles. Much better.
-- The AIS transmitter is dual channel, but only 1W. This is probably to minimize power draw and maximize useful lifetime. None-the-less, the range is probably only a ~1mi radius. And, maybe less -- if the EPIRB is floating in the water, the VHF is going to have a really hard time getting past swells. So, a very nearby boater (more likely ship) with AIS integrated into their MFD could find you this way. But, other than in busy waters, it may just help someone nearby look the right way. At least in my thinking.
Personally, I like the idea of it having AIS, but I am much more comforted by the internal GPS and 406Mhz signal. Well, that and the fact that it can self-deploy.