Being that my main niche in WAM is the wet/messy hair aspect, I voted long. Sometimes short can be okay, but generally I'm more down for medium length to long.
Noting that some folks never have to deal with what it takes to clean up long hair...I honestly don't care about hair length with the wam I like to watch as long as the head/hair gets messy. I'm super happy with my afro after years of shoulder plus length locs and mess both ways.
The aesthetics of long hair I do understand, but I also understand bless those beautiful souls when it's time to clean up. I don't miss the extra time needed to clean my hair.
I prefer longer hair. I'll admit that even watching my girlfriend wet her long blonde hair in the shower is a huge thing for me. I love beautiful hair on a woman and also love seeing it slimed, pied, watered and washed.