If you want a device that works on its own, even when your phone is dead or soaked, get the Garmin inReach Mini 3: it has its own touchscreen. If you'll always have your phone and want the simplest, cheaper device, get the ZOLEO. Both use the Iridium satellite network, which covers the whole planet, so coverage isn't the difference.
A screen, or your phone
The inReach Mini 3 has a colour touchscreen, so you can read and write messages without your phone. That matters when it's cold (phone batteries die fast below zero), wet, or when the phone is the thing that broke. The ZOLEO has no screen: you text through an app on your phone, and without the phone it can only send its SOS and a preset "I'm OK" check-in. For a weekend in Algonquin with your phone in a dry bag, that's fine. For a solo trek, the screen is worth it.
SOS
Both have a protected SOS button that reaches a 24/7 response centre, which then contacts search and rescue. Garmin's goes to its own Garmin Response centre. Either way, the button is what you're really buying: test the check-in message before every trip, and make sure your emergency contacts know what to do with it.
Plans: the part people forget
You need a subscription for either. ZOLEO's plans have a three-month minimum and a small monthly fee to suspend between seasons. Garmin has a low-cost pay-per-message plan and lets you suspend for free for up to 12 months, which suits someone who only goes into the backcountry in summer. Plans and prices change, so check both before you activate. The Mini 3 itself costs more up front; the ZOLEO is the cheaper device.
Battery and size
Garmin quotes up to 330 hours with tracking every 10 minutes; ZOLEO quotes 200 hours or more. Both are about the size of a deck of cards, around 150 g or less. On a long trip, the phone paired to them runs out first: a small 10,000 mAh power bank fixes that, and it's allowed in your carry-on.
Where you can't take either
Never take one to India. India bans satellite communication devices without permission from its Department of Telecommunications, and travellers, including a Canadian, have been arrested at Indian airports for carrying a Garmin inReach. A few other countries restrict them too, so check the Government of Canada's travel advice for your destination before packing one.
If you mainly hike in Canada and the US and have an iPhone 14 or later, its Emergency SOS by satellite may be enough. The older Garmin inReach Mini 2 also does texting and SOS with a smaller black-and-white screen, and is usually cheaper. The Mini 3 Plus adds voice and photo messages.