Closed hands, show that power!
Personally, I prefer closed hands since open hands can be distracting for me. However, I like open hands enough because it gives variety to the flexing pose.
A personal "peeve" is when the thumb is put "inside," or under the fingers and toward the palm...like a fist but one that would allow your thumb to break if you hit something with the "fist." The whole thing looks...awkward...you know...?