The height of the MRO mount you select depends on your preferences and the configuration of your rifle. My Colt has the standard carbine hand guard and a front sight base. I thought I wanted 'lower 1/3 co-witness' but the sight stuck up too far for my liking - I didn't have a 'cheek weld' that way.
So I went with the middle height, 'full co-witness' which turned out to be perfect for me. I have no need for 'QD' mounts. Now that its sighted in, I doubt the MRO will come off the rifle in my lifetime. The BUIS shoot fine through it.
So, with both sight systems zeroed at 50 yds, I have 'absolute co-witness', for what that's worth, which is basically nothing. You would never use the BUIS with the red-dot turned on, so it really makes no difference if the BUIS are sighted in at a different trajectory than the red-dot. Whatever, with the medium-height mount, there is plenty of room for the red dot over the front sight base so the tall mount really shouldn't be needed at all. I don't know how anyone could use the short mount on an AR even without sights. I know I couldn't scrunch my cheek down far enough to see through it.