reflex site(open style), simply because it's smaller and doesn't obstruct your view too much.
Believe it or not, i use my "open reflex sight" with the rubber protective cap on, i just cut up the part that's facing me ofcourse. You really don't need to see THROUGH the lense to be able to see the aiming reticle. By covering up the front of the lense you
#1 protect it from incoming BBs, and
#2 you effectively eliminate the RETICLE from disappearing among the background.
#3 you eliminate potential light refecting off the sight which may give away your position.
Ofcourse this only applies if you aim with both eyes open. Try it, it works better this way! .
You can't do this with a tubed style RDS coz sight allignment with the sight hole is MORE crucial then with the open lense types.