The 8AT has the advantage of being able to directly downshift up to 6 gears at a time (8th directly to 2nd for an extreme example). The 6AT must downshift sequentially (my biggest complaint of ATs and why I drove nothing but manuals for years).
I'm sure the 8AT software is still in need of refinement but it is the superior transmission to the 6AT (and in some ways superior to a DDCT). Driving my E90 335i & my parents' F30 328i convinced me of this.