I would have to disagree with this a bit. The iridium network is very reliable and there is inherent value in a SAR situation to two way communications. The GEOS operators can first let you know your mayday message has been received, then ascertain the nature of your emergency, then can coordinate with you for your rescue or other assistance. It can be critical to know that the rescuers are inbound an will be there within the hour and you should stay put until help arrives, or that the rescue the will be 8 hours out, and you should move the victim back to the hut as there is a real chance of hypothermia.
The iridium transmitter may be lower power (I don't actually know) but the iridium constellation is much lower than the GPS satellites that host most of COSPAS receivers, and is specifically designed for communications, unlike the COSPAS receivers which are hosted payloads with smaller antennas. We know our inReach is reliable and exactly how to use it, because we use it all the time - it is in tracking mode when ever we are out on an adventure. Professionally, I use Iridium phones and trackers in Antarctica, and have never been issued or carried a PLB when doing Antarctic or Arctic field work.
Given the choice, I would definitely choose inReach (or another Iridium based tracker) over a PLB.
I didn't choose one over the other. I view them as a little different and carry both. I decided that after watching Adam Greentree's account of an InReach SOS failure and then subsequently reading about others. I was also swayed by the Outdoor Gear Review article that ranked the InReach as their best satellite communicator on their aggregate review, but only gave the Iridium systems, including the InReach, a 7/10 for SOS. They gave both PLBs (Ocean Signal and ACR) 9/10. The cheapest Garmin, the Mini, is $350 (although I just saw them on sale) and the cheapest monthly subscription, limited to 10 messages, the one we get, is $15/mo. Over the 7 year lifespan of a PLB battery that never requires recharging, that equates to over $1600. The PLB on the other hand at a flat $289, works out to less than $3.50/mo. For that minimal investment, when it could mean life or death, I couldn't figure out a reason why not to carry both. I use the InReach, rarely, to communicate, mainly if we aren't on schedule or need non-emergent help. I would activate both SOS systems if life was on the line.