A Spot and an EPIRB are not the same, not even close.
An EPIRB, Emergency Position Indicating Locator Beacon is registered to a vessel (boat) and can be automatically activated when wet and out of its bracket. It must meet strigent standards and regulations on its battery life, tempertaure tolerances, and a whole slew of other stuff that most people don't care about.. until they need it. Any boater going offshore should have an EPIRB or at mininum a PLB.
A PLB, Personal Locator Beacon is registered to the person and is manually activated. Both the PLB and EPIRBs have a free registration. They work on the 406 Mhz signal, and most nowadays have GPS transmission capabilites when activated. The Cospas-Sarsat satellite system, is a global system that is in place strictly for emergencies. The 406 signal is sent to the Cospas-Sarsat system with digitally coded message. If the beacon has GPS, this information is also sent. Rescue can be alerted in as little as 3 minutes. The 406 signal gets Search and Rescue very close, GPS even closer and a seperate 121.5 MHz homing signal is emitted from the beacon. PLBs are lightweight, some float, some don't and have less stringent standards than EPIRBs, such as has to work for 24 hrs vs. 48. (don't know the exact requirements right off hand, so please don't quote me on that)
Spot is a tracking device and is linked by paid subscription to a private satellite system. When you push the 911 button it goes to a private company which becomes responsible for contating 911 and/or emergency services. Spot is good for those left at home with its email alerts and tracking, but in a real emergency, is not reliable. It does not have global coverage and transmission can be spotty, expecially in tree cover or canyons.
More information:
http://www.acrelectronics.com/faqs/spotvs.aspx