I dont know the Etrex, but I know about Polar, it uses air presure so any important variation in the pressure can give you imprecise reads. An interesting explanation it's here
http://support.polar.fi/PKBSupport.nsf/ALLDOCS/42256C2B001E0F6A42256B0B0034E39D?OpenDocument
As a general observation that I made from the use, Polar it's good measuring height differences, but if your base altitude it's wrong, the hole graphic mantains the error in a consistent way.
I would vote for the GPS technology.