The two packages are not alternatives for each other. They provide different things. Now that I looked at the package descriptions (see https://packages.debian.org/bluez-firmware and https://packages.debian/firmware-atheros, also for newer versions in bullseyey-backports) it seems that bluez-firmware is actually only useful for Broadcom and RaspberryPi chips, so that might not be on a Librem 14.
The firmware-atheros package is made specifically for Atheros chips, one of which is the wifi+bluetooth card in the Librem 14. The exact card might vary per model, but I also have a AR9462 from Qualcomm Atheros according to sudo lshw.
Maybe later I will test if removing bluez-firmware does not break things, then I will update my solution post above accordingly.