This is a tough one to call. I've never been a fan of The Sentry but I do believe he could defeat J'onn in a head to head battle. The prep time is a concern to Robert as the Martian can conceivably gain access to a Phantom Zone projector from Superman or some other BFR plot tool. However I have rarely seen J'onn be so proactive when he is in character. Even when he was distrustful of the rest of the Justice League he only made dossiers on the team's powers and abilities. He never actively worked against them.
As power goes I'd say The Sentry has the upper hand. He has physical stats to at least keep up with the Martian. His molecular manipulation puts him over the top (as much as people hate to admit it he did defeat and powered down Molecule Man). Even J'onn's telepathy is unlikely to help him as Robert has been able to resist Omega Level telepaths in Marvel 616. I'd say J'onn's only hope is if he can become Fernus. Then we would have a great fight on our hands.