having read around the forums the consensus seems to be that bad elf is not the way to go due to the risk of damage and also the need to keep the ipad in clear line of sight of the sky.
the choice is then between the gns 5870 and xgps140 - does not seem much between them - gns is about £30 cheaper. gns has a sliding on and off switch which could be a problem but no reports that it actually has been. other than here seen no comments on performance at altitude.