Unless you're considering KAC irons, there's not an optic worth owning for a serious use AR15 that is in the same price range as iron sights. So, if you have $400 to spend, buy an Aimpoint PRO and get irons later. If you have less than that to spend, buy some quality iron sights. For fixed, the Daniel Defense's are good and a good value. For folding, Troy, KAC, MBUS Pro...I suppose the plastic MBUS's are OK, but wouldn't be a first choice as a primary sighting system.
I agree. Plus with an Aimpoint Pro you can leave it on for three years straight before you change the battery. The argument I have heard is it is best to make sure you are proficient with your irons before you get an optic, that way you can still aim if your optic fails.