Depends on situation. If you're after trying to raise one stat, then 10%.
If you have really crappy weap/gear, a 30% could give you a 2 for 1 deal. You can raise the stat you're after and the attribute assimilation may raise another stat too...only recommended if you have crappy gear so you don't lose any nice stats already on the weapon.
Had a L35 bow that only had crit 4...it was +4 drop, so tried to up the crit.
Crit stone only went from 4 to 6, but the magic attack % went from 0 to 76% on attribute assimilation, something I was not expecting to see! Worked out nice, since my char was int at the time.
