Depends entirely on what you want to do.
If you want to learn how to fight your AR under adverse conditions that do not tolerate equipment failure, if you're going to spend hours attending classes and honing your skills, then spend more hours practicing those skills -- then yes, get irons and learn how to use them first. My choice would be the GG&G MAD because it is very low profile, locks in place, and keeps both sizes of apertures on the same plane.
HOWEVER...
If you want to plink with your AR, drill metal plates at 100 yards, blow up water jugs and melons, and just generally have a blast with your rifle in a safe range environment -- get optics first. You will not believe how much easier and more fun your rifle is to shoot with optics vs ANY BUIS. Target acquisition is very natural, you can practice your holdovers at longer ranges, and hey, if your optic fails (which is pretty darned rare with today's quality Eotechs and Aimpoints), you just pack up and go home, then send your sight off to have it fixed for free under warranty. If you want, you can buy a BUIS later and learn how to shoot irons (although you should still practice good cheek weld, consistent follow through, etc, even with optics).
Yeah, I know, opinions are like elbows...