It's based on base damage, not counting other DI.
So it depends on the weapon you're using.
If base damage is 13-15 (for a hatchet, commonly used in PvP for the faster swing speed), 30% can be an additional 3-4 points. Heavier weapons can yield 4-6 extra damage, but those are so slow nobody uses them.
Base damage for a 120 mage with 125 int and 120 eval for flamestrike is 44-47, so 10% of that is 4-5 points. But most mages aren't scrolled that high. FS is slow enough few people use them in PvP.
So explosion is closer to 32-36 damage, or 3-4 points.
So on paper they're about the same.
Unless you're talking about using magic arrow spam or fireball.