The key differences between the Interphone F5XT and Interphone F5S are the ability of the F5XT to charge via a 2.5mm cable which is included in the kit. The F5XT does not include a wall charger, whereas the F5S includes a wall USB charger. 

Also the F5S and F5 are not compatible with the F5XT firmware updater, however future upgrades will be released for both units.

The F5 and F5S are virtually identical although the F5S features detachable microphones and the F5 has the microphone attached to the left speaker.