Which one to buy? A two-way message function and a tracking function are nice add-ons. If you just want a straight emergency beacon, the decision is relatively simple: weight, battery life, replaceable battery or not, will it work where you’re going…but there are many devices out there which offer much more. Some (such as Spot) are one-way messengers, others two-way. Some offer ‘breadcrumb’ tracking which means they report your position (eg to contacts) every few minutes, so that you don’t have to ‘trigger’ them to have people realise you need help yet send emergency services to your exact location. It is not that hard to imagine a scenario where you are unable to trigger your epirb even if you aren’t dead! Some have maps, weather…all sorts of frills. Then there are essentially a number of different sat phone systems if you want to go the ‘whole hog’.
If an emergency beacon plus one-way messaging and light weight (and the ability to change batteries) are the main consideration it is hard to go past the Spot Messenger. It perhaps pairs well with an Iridium satellite phone as each uses a different communication satellite system so you not only have two emergency devices (in case one fails) but two systems as well. The new version of the Iridiium Phone (the Extreme: https://www.iridium.com/products/details/iridiumextreme) now has an epirb and other useful functions eg wifi hot-spot – and is much lighter 247 grams as against 375 grams! I must afford to update mine. Another interesting device is the Delorme Inreach SE which also pairs with your mobile but offers two-way text communication.
I think these two are my best options: Spot Messenger: http://www.findmespot.net.au/spot-gen3-gps-messenger/ & http://static1.squarespace.com/static/532cc334e4b0c8441ae7e9df/t/5376a2b9e4b095f55e73d717/1400283833538/SPOT+Gen+3+Brochure.PDF @ 114 grams and A$209 and Delorme Inreach SE: http://www.inreachdelorme.com/product-info/inreachse.php & http://www.inreachdelorme.com/assets/pdf/DeLorme_inReach_Brochure.pdf @ 191 grams and US$300.
NB: Iridium have a number of other products might suit you: https://www.iridium.com/products/types/Personal-Tracking
PS: I have always had some means of communicating (eg distress). I always let someone know exactly where I was going and when they would next hear from me/ should do something. Mostly I have had a 2-way radio which could access repeaters and/or a sat phone. I am just now thinking of adding a plb/epirb to the sat phone in case one fails (and the person at home triggers an unnecessary search). When Della and I are walking together we would each carry one so that (eg if one is swept away in a river crossing) the other can still call for help. I have hiked eg the Dusky Track in Fiordland NZ many times. Each year they fly out a number of bodies from there. I have personally called in air rescue to fly out bruised and bloodied folk on several occasions, folk who had no means of communication!