You can't test reliability but people with the right equipment (and connections) can. The tests are pretty conclusive that a PLB will work under almost any conditions in any terrain. The main PLB signal (406MHz) is 10 times stronger than a SPOT's signal. The PLB also has a secondary signal (121.5MHz) which is used by SAR, CAP, etc. as a homing signal. SPOT has no homing beacon.
Also, a SPOT is dependent on first getting a GPS signal, otherwise it cannot transmit your position. A PLB can still be used without a GPS signal, although the position is always more accurate if it can acquire a GPS signal. In other words, a SPOT is dependent not on one but two satellite networks. If either network is unreachable, you've got a nice paperweight.
PLB coverage is world wide. SPOT coverage is not. Again, SPOT is satellite phone technology. Look at a map of the coverage areas for sat phone providers. There are a lot of places that have no coverage, particularly in the third world or at sea. Probably not a big deal if all you do is hike in Europe and the US, but something to bear in mind.
The COSPAS-SARSAT network is run by governments worldwide and supports the maritime and aviation industries. There's huge support for COSPAS-SARSAT since world commerce depends on it. SPOT is a new venture by a commercial operation that reaches a fairly small, specialized audience. Will SPOT still be there in three years? Five? No one knows, but we can be pretty much assured that COSPAS-SARSAT will still be there.
The COSPAS-SARSAT network is a fairly large network with both low earth orbit (LEO) and geosynchronous earth orbit (GEO) satellites. It's fairly well documented that the satellite phone network is very thin in terms of the number of satellites. In some areas of the US, some satellite providers tell people what hours the sat phone service is unavailable due to "holes" in satellite coverage. Gotta wait to make that call until the next satellite comes into view. No such holes in the COSPAS-SARSAT network.
On the other hand, SPOT unquestionably has nicer features. The tracking feature is fabulous. The ability to send less than "emergency call SAR now" type messages is wonderful as is the ability to just say "I'm OK". Very reassuring to family and friends. Combine the tracking with the "OK" messages, and your family and friends know where you are and that you're OK throughout your entire trip. Can't argue with that feature richness.
So, if you want high reliability, low cost, and a service that is pretty much guaranteed to be there (short of WWIII or the second coming of Christ), then a PLB is the better answer.
If you want the truly wonderful features of SPOT, then you'll have to pay for them and make a significant sacrifice in reliability. And there's always the threat that SPOT may go out of business and your investment will be all for naught.
Personally, I chose a PLB, but in all honesty there have been a couple of hikes where I dang sure wished I had a SPOT so that I could let my wife know I was OK but just delayed.
Perhaps if you hike in a group or with a club regularly, having one SPOT and one PLB in the group would be the ideal situation?
Each to his own. HYOH,