They really are designed for different purposes, and you should consider which suits your needs better.
PLB:
- Personal (cig packet size) are readily available these days (I have one myself), water-proof can be had for about $350 (ACR resqlink).
- no update pings or transmission other than 'come and help me'
- only for life-treatening distress. I cannot stress this enough. It is not to get you out of a spot of bother. The national rescue coordination services will swing into action and send out a chopper or similar. see
https://beacons.amsa.gov.au/about/how-they-work.asp
- world wide. Will work anywhere. Land or water, and the local rescue services will turn up.
- originally a global government-coordinated marine rescue system (hence the AMSA link)
Spot:
- a private organisation. US based
- can send periodic pings to tell people where you are. Website lets your friends track you.
- can be pre-programmed with messages (see Ratbag's post)
- because of the above, is much more flexible. You can pre-program a 'need help, but not in danger' message, which your home base (receiving message via SMS) can either deal with directly, or communicate to the authorities. eg: if you're just stranded off the beaten track somewhere, they could send out a friend to help instead of burdening the rescue services.
- uses something called the 'GEOS alliance' to triage your distress signal and work out what to do. This looks like a commercial organisation, and I'd be worried about how they interact with the local Australian rescue services. Maybe they just pass on the message. On the flip side, they can coordinate private rescues.