Going to be personal preference really... but the advantage of lower 1/3 is the sight picture while using the dot is cleaner. Comes into play more when using the micro dot type sights that have a smaller diameter tube. Advantage to absolute co-witness is that you use the same cheek weld with the red dot as you do the sights. No need to look over the top of the sights like you do with lower 1/3. This really comes more into play when you have fixed sights, or you keep the flip ups raised all the time.