The Lowrie is soft and can't hit RHP stuff is a little stale. He got hurt once, played hurt which hurt his numbers, and then he got mono. Then he came back and put up a higher OPS than Marco did against RHP last year.
Right now, he's the better player. The kid should play until he proves he shouldn't. He's clearly got more talent, so if he takes the job and runs with, good for the Sox. Marco isn't going anywhere. If Lowrie falters, they've still got Marco ready to fill in.