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What is the difference between the $270 ones and the $350 ones?
Battery life and/or form factor. Longer life or smaller form factor equals more expensive, small form factor and long battery life equals very expensive.
Usually, on these types of units a dead battery is the end of the units service life
(battery costs more to replace than buying a new unit). So maybe spending more now saves money later by extending the time period between replacing the unit; but I personally doubt it as the cost of these things is coming down over time. The new one I'm looking at buying for example has a 7 year battery life, its battery pack is not a Hazmat concern
(i.e.: it could travel on an aircraft with me if I wanted to) and its half what I paid for my early model GME GPS capable PLB .
Look at the PLB's you are choosing between, and buy the one with the latest battery expiry date on it. You should find the more expensive units are useable for a longer period then a less expensive unit
(its not hard to find a 10 year battery life on some units now). But only you can decide if the additional time before replacing the unit is value for your money or not.
If its for your 4WD and you don't walk very far when travelling, then don't worry so much about the form factor. If your a keen walker though, a smaller form factor, less weight but more frequent replacement might be the right choice for you.
My GME is about to expire in two months time, so as I said above, time for a new one for me. My PLB that is about to expire gave me about four and bit years service, but thats just my recollection, not a fact.
Edit: typos, sorry.