Fowfee because of thumbs. To me, I could reach deeper with the fowfee grip. Also, jelqing is alternate hands stroke for stroke. With the fowfee, I would do several strokes with the right hand then switch and do several strokes with the left hand. I never threw this out for the guys to test so don't have any data other than my own.
Like many PE exercises, what works for one want work for another. It's like the "fowfer". My test proved it worked for "me". Some guys swear by it while it does nothing for others? A LOT DEPENDS ON THE DEDICATION OF THE INDIVIDUAL.
Good Luck.
fow