I think +magic damage on the sunder works better for bowlocks/characters that use a hex: bane buff, because the +damage enhances the damage of the hex at the application (just like how +fire enhances the damage of the buff at application). For other characters, like berserker barbs, +damage does nothing to enhance the damage of the skill.
In almost every case, -res is superior. Magic is a little different, however, in that with so few monsters being immune, the +damage ends up working similarly to -res, primarily because you can get +15% magic and only -10% enemy res.
I think in your specific case, because the bolts don't really do an obscene amount of damage as it is, the +damage on the sunder will be helpful. If you run into magic immunes that are taking too long to kill, you have other ways to take care of them, but the +damage will be slightly more beneficial for you in the majority of the fights.
i think +% magic double dips - for hex just like for enchant - once at buff, and again when it applies the actual dmg