The Sheraton is in the FRA terminal - you are 300 feet from the check in counters just across the overhead walkway.

The FS Marriott is downtown - figure 15-20 minutes to find your way and buy a train ticket, then 15+ minute train ride, plus a 20-25 minute walk if you know where you are headed. The street is separated along the route - park in the middle. If you have heavy bags a cab is likely best to get to the Marriott from the downtown train station.