There are a range of options designed, mostly, to help with box ticking and the like. If you want a system that is designed and certified to have the very highest probability of getting an emergency message out from anywhere on earth then you need a PLB. They don't allow you to upload your location to your mates or send messages but are designed as a last resort means of summoning help. There are several "pocket sized" versions on the market and there really isn't much to choose between them as they have to meet the requirements for certification so, basically, they all have to have the same actual performance. It is worth paying extra for a GPS enabled one as they see a much faster response time (around 8 minutes compared to about 45 minutes for the non-GPS version) and you can also get a version with a little blue light that confirms that your message has been received by the network. In the first instance the Coastguard (in the UK, in some countries it is the military or similar who monitor the network) will try and contact the "contacts" registered against the PLB so if you are working for a company or similar you can put their numbers against the device, if you just operate privately then you can put in friends or relatives who are most likely to know where you would be at any given time. Even if they can't contact anyone a rescue operation will be launched but they like to confirm the circumstances.
I suspect that some of the less robust systems are better for "box ticking" as they can send check-in messages at a fixed time period and this looks good on a risk assessment and I guess might be useful should you fall over dead and someone wish to recover the body. However, they don't have a stellar reputation for reliability, for some people they work well but there are some high profile examples where they've let people down, and are commercial systems so the quality of the people monitoring the network isn't as clearly defined as when it is the Coastguard. There is no certification process and so the devices may, or may not, be reliable and operating in a manner that means someone gets your message.
So depending on your perspective there are arguments in favour of both types of system. The bottom line is that if you want the very highest chance of getting a distress signal out in a very "last resort" emergency with the certainty that a fast and efficient rescue will follow beginning in minutes then a PLB is the way to go and it is certainly what I carry. If you are box ticking and like check-ins and, even, posting stuff on facebook etc. then one of the commercial services might be a better option and chances are it will probably work. My contacts know that if they hear from the Coastguard then I'm in big trouble but rescue is on its way and if they hear nothing then I'm either completely fine, or dead, so I don't need to be updating them every 10 minutes on my lunch and bowel movements.
