PLB output is supposedly 5W whereas InReach devices are supposedly around 1.6W (that's what the old Delorne units were, Garmin don't specify output). However they operate on different frequencies and reference different satellites in different orbits (Iridium is all LEO, CORPAS-SARSAT is a mix of LEO, MEO and geostationary, hence the need for extra output power) so any comparison vis a vis signal strength is outside my paygrade.
PLBs do reference multiple orbits of satellites which does theoretically help their 'time to ping' in less than ideal connections. Like an InReach though they too need to receive signal from the GPS satellite system in order to encode positional data into their beacon output.
It was interesting to note in a recent Tasmanian rescue the authorities mentioned they had some difficulty pinpointing the beacon due to it having an old (read: out of date) battery. Obviously shows how critical it is to keep the units in-date.
I don't think two-way communication is used to triage or otherwise prioritise SAR operations - as you say, they have a SOP they follow regardless - but the ability to receive first aid/treatment info and updates on rescue ETA would be invaluable in many situations. An InReach was used for the Kitchen Hut hypothermia rescue last December... the people looking after the sick walkers were able to receive treatment advice and updates with the rescue (which was twice delayed due to weather conditions).
Ultimately PLBs can be thought of a one-trick pony that do there one thing very well. InReachs do many things, mostly excellently. I think it any case where you have any doubt as to the efficacy of either device, take one of each. At least then when they report your rescue on the news, you'll be described as an "experienced and fully prepared walker"
