Co-witness Optics - your preferences

[hip55]

I am going to replace (then sell) my Elcan C79 3.4x28 optic with one that will allow me to use either the optics or the BUIS (Back Up Iron Sights) without removing the optic.

My AR carbine has flip up front & rear BUIS (YHM front & GG&G rear).

I'm researching the different optics and am considering an Eotech, possibly a 512 or an Aimpoint M4.

What are your preferences / experiences / recommendations?

Thanks