Slightly aft is more comfortable for me. I have read that the "traditional" ball-of-feet placement was developed by studying runner's kinematics. If you were to draw a horizontal line through my cleat, with the foot aligned vertically (toes up), then the line would cross just aft of the joint behind my big toe and right through the joint behind my little toe.

Doing that shift pretty much eliminated the ITB and knee issues I had been having. It also enabled me to drop my heel a little bit into my pedal stroke with greater ease - I "lead" my stroke ever so slightly with my heel, both up and down.