SPOT
Of late, we found the position upload reliable. Up to every 5 minutes with standard data plan, every 2.5 minutes at a premium. You don't pay per individual satellite message, only annual subscription, so you can send as many, or as few, position fixes as you want, the cost won't change. The only limit is the max tracking frequency (of 1 per 5 min, or 1 per 2.5 min as mentioned above).
When I say "reliable", this means that indeed every single one of the tacking messages sent are indeed received by the GlobalStar satellite network, and relayed back to the user (via smartphone app, or password protected webpage(. Not a single missed transmission over the past couple of months! That is better than advertised!
Also, email notifications (e.g., of Check-in OK) are unlimited and free. The only thing you pay for is when sending the same notification by SMS (to any GSM cell phone world-wide). Here the annual subscription includes 200 SMS. This initially sounds like a lot, but if you play around with it, say, notify 5 people by SMS, and send a message before take-off, and one at safe landing, you use up your 200 SMS in just 20 flight legs!
SPOT (Globalstar) won't tell you, or alert you about running out of free SMS. You just get charged at 'out-of-bundle' rates. The website is very user unfriendly, and it is almost impossible to understand how much of your free messages you have used up. SPOT just starts billing your credit card for excess messages, and even this not immediately, maybe only 6 or 8 months late, e.g. at annual renewal. So don't be surprised if you get hit with a Big Bill out of the blue.
Also SPOT's customer service / support desk basically is non-existent. Yes, you can send enquiries, but they will practically never respond. It's a disgrace!