I flew last year HKG-FRA-ORD in F on LH and also had an overnight layover in FRA. I opted to stay in town and was glad I did. As noted the train is easy and there are plenty of hotel choices near the Hbf. I stayed at the Savoy which was right across the street and very economical. It was nice to be able to walk around time a bit and over by the river which is a couple of blocks away.

I returned to the airport the next morning via the train and walked over to the FCT and schlepped up the stairs to the front door. It was a fantastic 5 hour wait. Next time I do it I will basically live in the cigar room....