http://en.wikipedia.org/wiki/Cospas-Sarsat#Rescue_statistics

Rescue statistics
In 2010, the Cospas-Sarsat system provided emergency-beacon alert data used in rescuing 2,362 people in 641 SAR events. From September 1982 until December 2010, Cospas-Sarsat alert data has helped to rescue 30,713 people in 8,387 SAR events.[5]

Aviation 20%
Maritime 56%
Land 24%
Total 100%
No detailed data on failed rescues.

What is interesting is the very low number of land rescues related to the very large number of PLB's.

IMHO - It relates to the very small number of backcountry users that carry beacons. Almost zero % of our customers have even heard of PLB's. Once educated on their usage, they still only understand and carry cell phones.

We monitor the airwaves in Idaho 18 hrs a day. Since the release of PLB's to civilians, we have not monitored one rescue yet. We carry PLB's because there are no idiots in the information exchange between the rescuer and us.

We have monitored many rescues initiated by cell phones and worried family members. If the victims were not dead, the stories are almost slapstick humor.

My favorite was a 911 call on a Sunday at 2:00PM. (155.520 MHz)
The dispatcher broadcast that a woman had called 911 to report that she was lost but that she knew where she was. The Sheriff requested an explanation. The woman stated she had a GPS but didn't know how to use it. She had gotten lost in her car and had placed the GPS on the hood. The woman gave her location verbally as " N43 Degrees, xx Feet and xx Inches and W115 Degrees, xx Feet and xx Inches. The dispatcher didn't know much more. The conversation between the dispatcher and the lost woman went down hill quickly.

After 15 minutes of eavesdropping, I asked the dispatcher to have the woman drive her car south and downhill until she hit the next intersection. At that time I had the woman read off what she saw on her GPS. She was only three miles from Idaho City, ID. I gave the dispatcher the location in deg.decimal degrees.

N43.88156 W115.83941

Her GPS preferences were set to Deg Min Sec which neither she nor the dispatcher understood. The only format that is VERBALLY understood is Deg.decimal degrees. Do not try to communicate with any type of chopper with anything except degrees and decimal degrees.
Now back to the original question – SPOT or the like or a PLB? You get to choose where to place your trust.