Scope has longer eye relief, but it also means the eye box has a tighter limit.
The higher the power the scope, the more sensitive it is to parallax. Those outrageous flyers you see in groups are the results of not maintaining the eye position. People cheat with moving the head a bit to get the recticle on target when their shooting positions are not perfect and they couldn't get back to the same spot over their breathing cycles , or/and the recoil moves the head a bit.
Fixed power optical sights like ACOG and Elcan are more forgiving than "scopes" in general when it comes to parallax. For a general combat sight, I will take the one that allows me to cheat a bit.