Stone Oracle teaches Armor boost, right?
Well, if you want to teach it to someone now, both have advantages. Sarah has more MP and she's faster than Random so she'll be the first to act in a fight.
But Random will get pre-emptive stuff (if you haven't already for him) and he's almost always in your party.
There will also be more persons who can learn this spell.
Anyway, it's just a matter of taste. It's not really important for the game. I myself didn't use it at all. I didn't use a single spell book until LF3 where I had to use 2 for a special person to win at the ArenA. I always had the same problem: who should get the spell? And since I could never make a decision I just carried all those books around until the end of the game.
