ACR and McMurdo are the two most common ones sold today. If you don’t need one that floats, then the McMurdo Fastfind 220 or the ACR ResQLink will both do. I prefer the antenna system on the McMurdo, but they are very similar in price, performance and cost of replacement batteries. (Which must be done every five years or so.)
Note that ACR, McMurdo and other PLBs (personal locator beacons) are the personal-sized hiking versions of EPIRBs (that are designed to float and are attached to boats and ships.)
PLBs use the public safety series of COSPAS-SARSAT satellites that cover the earth. It transmits a 406MHz signal that goes directly to the government rescuers. You ONLY activate it in times of life-threatening emergency. There is no two-way communication capability on most, and when you activate it, you will get ships, helicopters, snowmobiles, ATVs or troops to you in hours or days, depending on the weather. There is no subscription costs but each PLB must be registered and you MUST buy one coded to your country.
When you push that button, you must really need help because every possible resource is going to be mobilized to help find and rescue you.
The alternative is a satellite messenger system like the SPOT Messenger or the Garmin InReach. These use private, for-profit satellites and for-profit companies who will hopefully call the most appropriate response for you. You can also do messaging on most of them, and are possible to use just to say “I’m okay” or in event of an urgent situation but not an emergency.
The difference between SPOT and Garmin is that Garmin uses the Iridium series of satellites that are also designed for Iridium sat phones. SPOT uses the Globalstar series of satellites. Iridium covers the world, while Globalstar only covers major continental areas. (The U.S. military uses Iridium.) Few people in Canada rely on a Globalstar sat phone, even though it is cheaper, and no one in northern Canada uses Globalstar.
Quite frankly, for all the above reasons, I would never trust my life to a satellite messenger. Satellite altitudes are lower and performance is hit-and-miss. I use a PLB.
Satellite messengers are very expensive and subscription costs are horrendous. Plus, being for-profit companies, they are primarily focused on billing your credit cards whether you want them to or not. SPOT has a particularly bad record of poor customer service, cancellation policies designed to trick you into paying every year and few ethics in what they will do to keep billing you. They have even charged credit cards that have expired by creating their own new expiry dates and not telling the customer. You cannot cancel easily and they charge your card a month after your expiry date, giving users a VERY narrow window to cancel.
I always wonder how focused SPOT will be on rescuing you when their billing policies and customer service is so bad.
As my friend also says above, trust your life to a PLB, not a two-way radio or a satellite messenger.