Armed with an assault rifle with 50 clips of ammo and a short sword...

Would you rather...

Face a horde of zombies

Or

Face a horde of orcs

Edit: There are 500 of them in either horde

Depends on the numbers for a specific answer, but probably the Zombies: They're generally slower, generally unarmed, generally unarmored, don't use weapons, and don't use tactics.

^

7691925
Zombies can easily funnel them

Orcs. I don't need to worry about where I hit them, I only need to hit them. psychological (fear) works when you begin mowing the lawn, Metal armor is ineffective towards guns, their weapons are majority melee, though their crossbows would be an issue. That said I feel there is an actual chance against the orcs where the horde of the dead won't stop the orcs will pause, and the zombie can take bullets, but orcs cannot.

7691970
Let's say that there are 500 of them.

Orks, they have the capacity to fear, and strategize, something you can use against them. Zombies on the other hand will pursue you relentlessly, mindlessly, and heedless of the casualties. One little scratch and you're dead.

As someone who has faced both, I'll take zed's any day. Zed's can be kited easily, where as orcs take more energy to fight. Zed's are slow moving and easily distracted, orcs are smart and move just as fast,if not faster than my fat ass. Plus even though zed's need headshots, I'll have more time to aim with them that the orcs. (I LARP, so I have faced both orcs, zeds and fast zeds.)