I don't have a strong preference, but I do like Nature Sprites a little better. Harvest Sprites sounds too limited, to me...like they only have anything to do with the harvest, but at no other point. But that's just my odd little hang-up.
Ironically, I think the limitation of calling them Harvest Sprites is what makes me prefer the name to Nature Sprites (nostalgia aside). The name Nature Sprite is super generic - not bad, just generic. There's very little indication of what a "Nature Sprite" is, of what its purpose is. Heck, a Harvest Sprite could be a type of Nature Sprite.
If the name Harvest Sprite can't be used due to some trademark issue...I'd prefer something different to either.