Actually there are differences
Spot is a commercial venture and as such sells the safety concept. They are a great product and I have one. I like its tracking capability as a starting point in aviation SAR. But, it uses Globalstar satellites (the same Globalstar that offers Satellite phone service) for tracking. When you activate the 911 feature it is monitored by Spot who then calls SAR. They have had a few "misses", but they are evolving quickly. In short, it is a good product but it is not the "same" as a PLB.
PLB's OTOH are a product that uses the official SAR COSPAS-SARSAT emergency satellites that is funded by the taxpayers and is globally monitored by SAR personnel. It also is built to international standards and has a homing signal ability that the Spot does not have. If you activate a PLB it is monitored by SAR personnel with no middleman and its a gold standard for rescue - thats what its legislated and designed to do.
Finally, costs. The PLB's prices are falling fast - $400 or less now. They need a new battery every 5 years and otherwise cost nothing. A Spot needs 1) to buy it (Sometimes they have offers - mine was free with a subscription). 2) you have to buy a annual subscription (99 or 149 a year - the extra is for tracking). 3) Batteries - a Spot uses Lithium AA batteries that are good for (in my use) about 6 months. A PLB has a 5 year replacement guideline (doing mine now - looks like just under a $100).
I think if you average the costs over time to see which costs less to operate, the PLB has a greater up front cost, but a lower cost of ownership over time.
Good luck, they are both good. But this may help make a informed decision.
There is a very good comparison
http://www.equipped.org/SPOT_ORSummer2007.htm here that tells the difference.
I have a PLB that I bought a few years ago. I think if I had to carry one, the PLB would be my choice. I carry both. Good luck.
The PLBs and SPOT use the same satellite technology, so I dont think there is a quality difference with respect to their message sending capabilities.