Small World, what does "permanently discard" mean?
Because it says "back into the Storage tray", it is the first option; Skeletons and Sorcerers can reuse them. Note the limitation in the rules for both Skeletons and Sorcerers: 


  If there are no more tokens in the storage tray...


Both Skeletons and Sorcerers state that the new unit comes "from the storage tray", and the rules for the loosing player state that the killed unit goes "to the storage tray".