This is a difficult issue. Partly because we don't really have enough information to make a proper decision and partly because we don't know if its the right decision until its too late!
There is, however a significant body of data that says ELT's are very unreliable in their activation. There is no definitive piece of work, but a LOT of smaller pieces of information. It might have been a piece by the US AOPA that convinced me, I forget. Try this
ELT, Phone Home - AOPA
You'd think that a proper analysis of ELT function might be a worthwhile use of time by CASA or the ATSB.
The reasons fixed ELT's don't work include accident damage of the fixed antennae, damage to wiring, the ELT breaking free from its mount and plain old just didn't work (off topic, but there was no ELT activation for MH370).
But we like the idea that there is a guardian angel in the tail that will bring help magically.
The advantage of an EPIRB is that a) it will work and b) it can be initiated in the aircraft on the way down before you crash land.
One of its weaknesses is that they frequently reside in our pilot bags and we tend to forget to brief passengers on them. I'm beginning to like the idea of a second EPIRB velcro'd in the back for the club seated passengers.
Its a well worn soapbox of mine, but we should all be monitoring 121.5 in cruise when the second radio is not doing much. Australia is nearly the only country in the world where the international distress frequency is not monitored. Boyd Munro wrote a piece 10 years ago that is still relevant
CAN IT EVER BE APPROPRIATE TO MONITOR 121.5 EN ROUTE?
The sad truth is that more and more we are on our own because the safety bodies are deserting us. Now satellite phones and satellite trackers like spot and spidertracks need to be part of this conversation also.