Personally, I don't believe in folding rear or front sights, that is if the rifle is used in a defensive role. If the optic goes down in a defensive scenario, you really won't have time to flip up your iron sights. With the standard fixed sights, all you have to do is lower your head and there they are. Also, in theory, the mechanism of flip-up sights could break, jam, malfunction and leave you with a useless sight system and no re4al way to aim if your optic is also down. The event may be unlikely, but if your life depends on your rifle and what you choose to mount on it, I would prefer to have as little on it that could go wrong.
However, if it is a target/plinker rifle, then the folding sights do look pretty cool. Also, if you use an ACOG, magnified optics, or night vision of any kind, the rear sight will have to be a flip-up type as theose optics won't clear a fixed rear sight.