They are not the same so they really can't be directly compared. Grounds of the Orc Strategist makes Orc Strategists, which are a heavy ranged combat unit. Orc Nests make orcs. Orcs are a resource, you use them for stuff like negotiating, building upgrades, and unlocking tech advances. And in the Orcs chapter, they are needed to make guest race goods. You can also use orcs to train Orc Strategists in the Training Grounds building.