There was a fairly in-depth discussion about this earlier, here it is: viewtopic.php?f=3&t=3023&start=10
At its most basic, the Orc's +2 Base damage is felt a lot more at early levels, whereas the Human typically comes out on top at higher levels; 10% bonus attack at level 10, with no extra bonuses, is 5 bonus damage.
Of course, there are many extra factors that affects the comparison, and the general consensus seems to be that there are many situations where Orcs will be flat out better. This especially pans out in focused strategies involving things such as knockback or items that already give %damage boosts, as well as prepping extra attack boosters and so on.
Keep in mind, of course, that really leveraging the Orc's potential often means heavy conversion early in the game.