I am going to build an M4 flat-top on an 80% receiver.

I want to mount an Aimpoint up top, and need to decide whether to get a standard front sight or gas block.  Does the front sight interfere with the sight picture thru the Aimpoint?  Am I better off going with flip up BUIS front and back?  I guess the only advantage of the fixed front sight is that it would save some money.

I'd go with an RASII, LMT BUIS and standard front sight. That should allow you to cowitness thru your aimpoint and put your aimpoint in the best position--and should take the reddot out of FOV of your backup sights.

You could also go with flip-ups front and back(ARMS #40) and a riser on your flattop(or not).

I don't have an AIMPOINT but that is what I know about their set up. Others can chime in obviously

I put an RAS II on my M4gery and have since put on a 16" M4 barrel with an ARMS #40 BUIS. I plan to put an Aimpoint on the hump of the RAS II, which is the ideal place to mount it, using a LaRue low profile mount (which is perfect for that hump). I hope to finally purchase the Aimpoint and mount along with a CCG side sling adapter for the FF rail in another 3 weeks to complete my rifle.

Here's a picture of mine:

I"ve got the Eotech on mine and I lowered the front site.The red dot sits in the center of the v just like a lolly pop.Works great if you want to consider the Eo.