We leave Frankfort at 12:20p.m. and want to know if there is a better place to spend the night before we leave. We will be in Rothenburg the night before that.
If you're concerned about catching your flight, and you want a nice, inexpensive hotel at the airport, check out the Accor chain. Their Ibis Frankfurt Airport can be had for as low as 49 Euro per night, depending on when you're booking. We've used Ibis many times, and have been quite happy with them. Check http://www.accorhotels.com/accorhotels/index.html, type in Frankfurt and your date of arrival, then look for the Ibis Frankfurt Airport.
In the past we have stayed in Worms and Mainz before our return flight from Frankfurt. They were both quick trips to the airport. I prefered Worms which is a little further but still only about 1/2 hour drive away from the airport. The Dom Hotel is a good choice, in a pedestrian area.
I don't know how much of the day before you have to dedicate to travel and exploring, but I would recommend staying in Bacharach and exploring the Rhine a bit the day before and then traveling in to Franfurt to leave. That was what we did, and Bacharach turned out to be one of our favorite places. Letti and Pension Letti was fabulous!