Price Differences
There are no price differences between the Orc and Human units. The only price differences between Orcs and Humans are the Blacksmith upgrades. For the Humans it's 800 gold, 2400 Gold for offensive upgrades in the Blacksmith while for the Orcs it's 500 Gold 100 lumber, 1500 gold, and 300 Lumber. Human offensive upgrades are more Gold intensive. This is handy on the few maps where there is more Gold than Lumber such as Nowhere to Run and Oil is the Key. Orc offensive upgrades are made up of more of a mix of Gold and Lumber which some players like better.

Overall Differences Before Castle or Fortress
Humans and Orcs are virtually the same until the Castle/Fortress Stage with the Blacksmith offensive upgrades being the key difference.

Castle or Fortress Stage
Once you hit Fortress/Castle, that is when the Orcs and Humans head their separate ways due to the different spells available to them.

Orcs Rule the Ground War After Fortress
Once the Orcs have completed their Fortress, Altar of Storms, and have researched Ogre-Magi and Bloodlust then that is when the ground battle turns in favor of the Orcs. Bloodlusted Ogres do three times the damage of a fully upgraded Paladin. Bloodlusted Ogres can also quickly and easily break their way into enemy defenses and tear down Towers. Paladins receive Healing but this can often only help them after a battle is finished.

Once the Humans reach Castle they Humans have to fight more defensively. They can no longer beat the same number of Ogres with Knights. They have to have more Knights or something else such as Mages and Demolition Squads to help them in the ground war. They must use surprise and hit and run attacks to catch the Ogres when they are not Bloodlusted.

Which to Choose?
It doesn't matter which race you play if the game never reaches the Castle/Fortress stage.

Control
The Humans require a bit more control ability in the Castle stage as they must use extensive use of Mages to combat Bloodlust as well as fight Bloodlusted Ogres using hit and run tactics using Paladins and Healing. Orcs however often only need to Bloodlust a few Ogres as their strategy sometimes making the use of Death Knights. This predictability can be their downfall against highly skilled Human players.

Mages vs Death Knights
Mages are far more capable than Death Knights. Mages have more spells that can be used in a variety of situations while the majority of Death Knight spells are trick spells. Death Knights main spells are Death and Decay, Haste and Death Coil with the last two only useful in some situations. Mages on the other hand have Polymorph, Blizzard, Invisibility, Slow which are always useful and Flame Shield for certain situations. Additionally, Death Knights are vulnerable to Exorcism. Since Human players often have plenty of Paladin's they almost always have enough mana to cast Exorcism on Death Knights. You can really deny the Orcs the ability use of Death Knights through Exorcism.

Humans Are Powerful On Sea
Humans are a very powerful enemy on sea maps that reach the Castle/Fortress stage. While Orcs can use Bloodlust to power their way into enemy islands Humans have a few tricks available to them which are virtually uncounterable by the Orcs. These tricks include Invisible Transports, Invisible Mages that cast Blizzard on the Peons, hit and run Polymorph and Exorcism attacks. Orcs can often do little to defeat such attacks other than direct invasions or taking the Human player off the sea which becomes hard to do if the Human player has Transports, Invisibility and Blizzard. Humans can also use Gryphons for hit and run missions then return to their island and Heal. While Orcs can use Bloodlusted, Hasted Dragons for hit and run missions, Mages can quickly cast Slow and Polymorph them. Humans have the ability to put up some major defenses on their island using Flying Machines and Mages, as well as provide some serious offense into heavily defended bases using Invisibility.