This one's tough. They both have good matchups with good receivers and a strong running game. Semian seems to be the more consistent option of the two, but Prescott has the higher upside. It's up to you which you choose though.
I love Marv but you do realize OBJ is on pace to be the greatest WR of all time right? I'm saying dude is averaging like 100 yds/gm and almost 1 TD/gm. Higher than Marv, Rice and Moss. I'm just saying. Forget dudes attitude and hair do and persona, he is putting up unprecedented numbers from the outset of his career. For a receiver that is rare.