I first bought a PLB and carried it on trips in remote areas. More recently I acquired an InReach Explorer. Friends have the InReach Mini, so I have some knowledge of it as well. I have no experience with the SPOT, so can't talk about that.

For only sending an SOS, the PLB is the (slightly) best choice. It can get your location using GPS, and send that back to the RCC (Rescue Coordination Center). In the US, the RCCs are run by the Air Force for land areas, and the Coast Guard for marine areas. The RCC will then contact local rescue assets. Besides getting a GPS fix on your location, the PLB has the redundant capability to get a less accurate but still useful location using doppler from LEO satellites. This capability might be useful if for some reason a GPS fix can't be obtained (for example under heavy tree cover). The doppler position fix is a legacy feature from the pre GPS era. The doppler fix takes several hours to obtain, and is accurate to perhaps a mile or so. The PLB also has a low power homing beacon. The PLB broadcasts a robust 5 watt signal. The PLB is the cheapest, with no ongoing subscription fee. The downside of the PLB is that it ONLY sends an SOS with your location. There is no way to send any additional info such as the nature of your emergency. Also, the PLB has no capability to send routine texts to family and friends. And the PLB is not usable as a stand alone GPS navigation device.

Lately, I have been carrying InReach Explorer on my trips. The Explorer also gets a GPS location. When the SOS is triggered, the GPS location is transmitted (at ~1.6 watts if I remember correctly) to the Iridium satellite network, which then sends it to the International Emergency Response Coordination Center (IERCC), which is now owned by Garmin. The IERCC will then contact local emergency responders. The InReach has no back up doppler capability, so if it can't get a GPS fix, it can't send a location with the SOS. However, in practice, the InReach devices seem to have a good record of getting SOS messages out. In an emergency, the big advantage of the InReach over the PLB is that it allows two way text messaging. When you trigger an SOS the IERCC will immediately text you, inquiring about the nature of the emergency. You can respond with details about your predicament, and what help you need.

A further advantage of the InReach is that it allows you to do routine texting with family, friends, etc. When I do a solo trip, I try to send my wife periodic messages to let her know I'm OK, and what my plans are. If I'm going to be delayed a day or more by weather or other conditions, I can let her know so she doesn't worry. You can set it up to send periodic location messages, so family/friends can follow your progress.

The InReach is also a full functioned GPS unit, which you can use for routine navigation. The interface is not quite as nice as newer designs of dedicated navigation GPS units, but it works adequately. You can pair your InReach via Bluetooth with your smart phone. This makes texting easier and adds some navigation capabilities.

The downside of the InReach devices is that the initial cost is high, and you MUST maintain a subscription. The subscription cost varies with several options. I've gotten by nicely with the basic "Safety Plan" for $11.95/mo.

I don't have an InReach Mini, but several friends do. It is much smaller and lighter to carry. Battery life is about half the larger InReach Explorer, but still adequate. Because of it's tiny size, trying to send messages directly from the Mini is a real pain. You definitely want to pair it with your smart phone.

One final tip. If you decide on the InReach Explorer, be sure to get a new one made by Garmin. The original Explorer, made by DeLorme before Garmin acquired it, had a serious design flaw in the button to turn on the SOS. There may be some used Delorme units for sale on ebay, but don't buy one! On the Delorme devices it was possible to accidently trigger an SOS if the device was jostled around in your pack. This happened to a friend of mine, but fortunately she was able to cancel the SOS before a rescue was launched. Garmin has fixed this problems on the ones they sell now.