Satellite messengers

The network is usually the same. What differs is the response chain behind the button.

Consumer satellite messengers in this category generally run on Iridium, a low earth orbit constellation with genuine global coverage including the poles. So comparing them on coverage is mostly comparing them on nothing.

What actually differs

The response chain. Pressing SOS does not call the local rescue service directly. It reaches a private coordination centre, which establishes what is happening and then contacts the relevant national authority. How good that centre is, and how well it knows the country you are in, is the most important difference between devices and the hardest to judge from a spec sheet.
Addressing. Some devices give you a dedicated number and address, so the person at home replies with an ordinary message. Others route everything through the manufacturer's app. That sounds trivial until the emergency involves someone who has never installed it.
Standalone or tethered. Some units have a screen and keyboard and work with a dead phone. Others are a relay that is useless without one. A phone in heavy rain at low temperature is not a reliable input device.
Tracking interval. How often a breadcrumb is sent, and whether it costs extra, varies a lot and drives both battery life and subscription cost.

Phone satellite messaging is not the same thing

Several phone manufacturers now offer emergency satellite messaging. It is a genuine improvement over nothing and it is not equivalent: coverage, message throughput and whether it works at all vary by country and by handset, and it depends on the phone surviving and holding charge. Useful as a backup, weak as a plan.

The subscription is the product

The hardware is a one-off. The plan is forever, and plans change often enough that any specific figures here would be wrong within a year. Check current terms directly, and check in particular whether the plan can be suspended out of season and what reactivation costs.

Not a substitute

A messenger shortens the time between an incident and someone knowing about it. It does not shorten the time before help arrives, which in Norwegian mountains can be many hours in poor weather. It is an addition to leaving a plan with someone, not a replacement for it.

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