It's your call but it's meant for looking down the top of an AR stock and co witness with the reciver. If you have any drop in your stock you head will have to come off the stock and you may get your cheek slapped so to speak by the stock.
The lower should be closer to co witness with the gun and will give you a closer point of aim point of impact at close and far distances. Always mount optics ask close to the bore of the weapon as comfortably possible.