I would go with both rear and front flip sights; but would mount the front sight on the barrel instead of the rail. When you apply pressure onto the hand guard (say in a roll over position or when using a car or wall for stability) your changing how your front sight (if it were attached to the rail on your hand guard) is in relation with your barrel...thus changing your sight picture and then of course actual shot placement. If the front sight is on your barrel, it doesn't matter how your hand guard is, your sight is ALWAYS aligned with your barrel.
Your right - it does take about a second more to flip up both sights if you went that way. That's not something I would worry myself with, but may be of concern to you.
Tactical advantage: Sturdy sight, don't have to flip up if optics go down.
Disadvantage: Could get caught on things like brush (if you use it outdoors like that), gets in the way of the optic reticle.
You could always get an open top front sight, which would eliminate it interfering with the optics.