Common wisdom holds that orcs are better levels 1-5 or so, while humans are better afterwards. I was curious to see the exact numbers, so did a bit of math. The results were surprising!
Assuming level 10, with no items in inventory, 400 CP (five conversions for the orc) gets the orc 10 damage, while it gets the human 20 (four 10% attack boosts, times 50 damage base). Factoring in attack boosters, the Orc gets 13, while the human stays at the same 20. Extrapolating, the orc equals the human with 100% attack, and from any point after that is going to do more damage. 100% attack isn't even that hard to do - extra attack prep gets you 50%, Rogue and Barbarian get 40% for free, and the Warlord gets a whopping 60%. Throw on Dwarven Gauntlets (nearly an auto-prep for me) and the perseverance badge, and you're easily in the 120-130% range. And remember, this is at level 10. Going lower level is only better for the orc.
In conclusion, for high %damage classes like Warlord, Berserker, and Rogue, orc is always better - even in the super lategame.