We had that discussion. SPOT signal in case of 911 call will go to SPOT's private response center, that will in turn contact appropriate authorities. It does not go to "your contact" as previously mentioned. While it does not go to government run center, it does go to people dedicated to the purpose that will make sure someone will be sent to rescue you. The center is ran by search and rescue company and additional SAR insurance is sold pretty cheaply with SPOT, giving you additional rescue options beyond government help.
SPOT also gives you real verifiable service, which you can test and see really working anytime. With EPIRB you have to wait till real emergency to find out if it works, which probably isn't a good time to realize you have a faulty unit (and no, "self check" is most certainly NOT a valid test - I can tell you that as a software engineer).
So, personally if I had the money - I'd have both anyway - why not. However, since I am not filthy rich - I choose SPOT.
Note: SPOTs satellite coverage is not as comprehensive (it is Globalstars *data* link coverage) and if I planned to sail outside SPOT coverage area, I would choose EPIRB. However, since that is absolutely impossible as I don't plan to sail in extreme lattitudes - it's a purely theoretical note.
A sensible post above amidst a somewhat distorted picture otherwise. What we're really discussing here is a difference in rescue location philosophy. Contrary to some postings, there's no one right choice for everyone.
There was a day when we did not have EPIRB's or satellite communications for that matter. Vessels commonly reported in every 48 hours or so, depending on their location and the methods of communication available.
Our initial consideration is that we will want our rescuers to know precisely where we are at our moment of distress. Not at all a bad idea but one that, with an EPIRB, comes at a price; lack of knowledge about our location and progress prior to the distress moment. Consideration must also be given to brak's point as to whether the damn thing works when we need it to, or if we're able to deploy it at all. These are just potentialities as, the one thing we know for sure, we never know what will happen in an emergency situation until it is upon us. We do pay a hefty price for this on demand service.
The Spot service offers what might be considered a more traditional form of vessel tracking. In the same way that a ship's owners like to know where their vessel is at, we can presume that the ocean voyager, the nearby coastal, and the solo sailor have someone concerned about their whereabouts and safety. For a fraction of the cost of a one time use EPIRB they can outfit with a system that allows them much more usable information. The only question being it's utility in a distress situation.
Given that the average sailboat will be hard pressed to cover more than 200 miles in a day, if that, the calculated risk of not having a real time accurate fix on it's position would seem to be less urgent. If the Spot is spot on it's all to the good. But, if all we have is a twenty four hour old position fix, that's not chopped liver either. And a functioning Spot will have also given us the vessel's course made good as well.
Certainly, cost being no object, one can take a suspenders and belt approach to this. For those on a budget though and desiring to achieve a reasonable level of safety in terms of being located, the Spot may well fill not only the rescue need but offers non verbal communication with those concerned on shore. Money spent in one area takes money away from another area. If real trade-offs are to be made, I see no reason that a justification for using only the Spot versus a real EPIRB can be made to the benefit of, say, carrying a liferaft or affording a sat phone, etc...
Of course, if you acknowledge that going to sea is an inherently risky business the risk of which should be borne solely by the risk-taker you'll need none of this stuff when you cast off.