I carry the SPOT AND a 406 PLB both. The SPOT is a handy tool to let the better half know I'm parked somewhere without cellular service, and okay. She has firm instructions, however, not to panic if she DOESN'T receive such a message. Sometimes the SPOT takes a few tries to get a message through.
Also, I like the tracking feature of the SPOT. In the event that something really bad happens, and I just flat disappear, my wife has access to the SPOT tracking page, so she can see where I was moseying, and hopefully let S & R know where to look for my carcass.
There may also be a situation where I need assistance, but I'm not really in a life threatening emergency. I can use the SPOT to send a "distress" signal (NOT the 911 feature), again to my wife, who hopefully will send friends to assist.
On the other hand, if something really bad happens, and I have an emergency, I'm going to punch the button on the Micro Fix PLB, and alert the cavalry to come get me. Those are the guys (RCC) with ALL the tools to rescue me, and I want them involved, rather than take my chances on the SPOT folks to connect with the right resources.
So, in my opinion, you really should carry both. I sure wouldn't bet my life on the SPOT system. One of these days, we're going to hear of a story where someone pushed that 911 button, and the SPOT folks weren't able to get the message to the right people. I think it's bound to happen, and if I was the pusher of the button, it'd probably happen on my watch.
On the other hand, SPOT has a lot of really nice features, and for the price its well worth carrying.
Just don't forget your PLB.
MTV