Which would be better for a feral druid
doing what? My $.02 is that for a feral druid doing cat form DPS,
Assassin's Stone with it's +108 AP would be better than the regular old Alchemist's stone with its +45 AP and some crit thanks to the +15 to str and agi.
For a feral druid is tanking in bear,
Guardian's Stone would be a great way to pick up loads of Def rating (+54?!?) so you can rejigger other gear toward stamina and agility. Or you could gear Assassin's stone for threat holding as you point out, or regular old Alchemist's stone for 45 AP, some crit, and some health.
For a feral druid that healing (sorry, gotta make the whole "we do what we gotta do despite spec" point) then
Redeemer's Stone would be best for the +healing. : )