The air/oil seperator filters the oil vapors out of the air and feeds the filteres air back into the intake.
Where as a breather filter simply vents the filtered air into the atmosphere.
In the stock airbox there is a piece of foam that "acts" like a air/oil seperator, but the foam doesn't filter or flow well.
Its really just a personal choice if you want a air/oil seperator or jus ta breather filter.
Although some tracks (if you drag or auto-x) will not allow atmoshpere venting of anything.
