Which results in more damage when hitting an enemy from behind, the +7.5% attack from behind bonus on swashbuckler gems, or the +7.5% base damage from bandit gems? Obviously the bandit gems are more desirable since that bonus works all the time, but they're also a lot more expensive, and I'm making a bludgeon build with extended stun jewels, so I'll also be attacking from behind a pretty high % of the time. I'm always scared of bonuses that I haven't tested, because maybe a +5% xbonus turns a +5% stat into a +10% stat, or maybe it turns it into a +5.25% stat.
I think the only way to figure this out is by testing with practice dummies. I suspect base damage and attack from behind math work differently.