In general, InReach has had a very good success record for SOS use. Certainly a lot of successful rescues in the SW canyon country. Unless you are planning to traverse some really narrow and tight slot canyons, I think an InReach would serve you well. (Usual disclaimer, no pecuniary affiliation with Garmin or PLB manufacturers.)

The first issue is getting a GPS signal in to the unit. I've done some GPS trip logging while hiking in canyons and valleys. Sometimes, in really narrow (as in a couple of meters wide) slot canyons, the GPS will not get a signal at all. But usually it will log my location. What does happen a lot is what is called "multipath error", where the signal bounces off a canyon wall before hitting the receiver. When you display it on a map the location chatters around. Location errors I've seen appear to be up to about a 100 meters or so. This will effect either a PLB or an InReach.

The other issue is getting an SOS signal out to the satellite. In theory, the higher wattage PLB signal might help, in a few specific situations such as heavy foliage cover. I doubt that the extra wattage will help if you are in a narrow slot canyon. In practice, the InReach seems to do quite well for SOS signaling. I know of no side by side tests, or anecdotal info from SAR that would bear on this, so the difference is purely theoretical to the best of my knowledge. Both devices will continue to transmit till their battery dies. Unless the sky is totally blocked, sooner or later a satellite will come into view and the SOS will get out.

I do have some experience in sending routine texts with InReach, in widely varying terrain. It sometimes takes awhile for the message to go out, perhaps up to 10 minutes or so, but the signal eventually goes out. I've used it in some pretty rugged terrain, but not so far in any slot canyons.

In the event you do need to trigger an SOS, the advantages of two way text communication can't be overstated. You can inform the rescue team of the nature of your emergency, and seriousness of your immediate situation. Consider two hypothetical emergencies. You are in rugged country, it's 2 hours till sundown, and the weather is deteriorating. In the first scenario, during your hike you've experienced sudden chest pains and shortness of breath. It feels like an elephant is sitting on your chest. In this case, you need immediate high level medical attention, and the rescue authorities may consider a dangerous helicopter flight in bad weather and marginal light. In the second case, you've broken an ankle and can't walk, but have managed to improvise a shelter. Your situation is reasonably stable for awhile. In this case, the rescuers might decide to delay a flight till morning and better weather. Or they may opt for a slower land team to walk in and carry you out.