Here's my vote:
I have BOTH a SPOT and a 406 PLB.
I do not trust the emergency feature of the SPOT to get me help. On the other hand, the PLB will alert RCC, who has ALL the resources appropriate to SAR. If I need rescue, I don't want someone calling the local sheriff's office. I want someone who has ALL the resources, and knows who to call. No offense to the SPOT people, mind you--my point is specifically that this is RCC's job, and they do it well in my experience.
On the other hand, the SPOT offers TRACKING, which as noted by another poster, could help to locate you if you DIDN'T activate the 406 beacon. If that's the case, it might just help to expedite the carcass recovery, but that would make things easier on loved ones in any case.
I also like the "I'm okay" message function of the SPOT, though again, I would NOT rely on that as any kind of guarantee...just a "makes my spouse feel better" kind of deal.
So, in my mind, the SPOT and the 406 PLB serve two quite different functions, and both have value. The SPOT is relatively inexpensive.
If you're going to have one or the other, though--buy a Rescue-Fix PLB from these folks:
http://www.life-raft.com/product.asp?Pr ... =12052&l2=MTV