The only reason to stay at the Kempinski is if LH is providing you with a free room (offered for a paid F ticket). There is a nice swimming pool there which is worthwhile (I find swimming to be the ultimate remedy to being "cramped" into the somewhat uncomfortable F seat on LH). Otherwise stay in the lounge (there are some recliners in a darkened area of the lounge). The only problem with the lounge is severe over-crowding at times, otherwise it would be good. My own choice would be to take the train to Mainz and either sight-see or book a room at the Hyatt in Mainz (much, much nicer than the Kempinski and about 30 minutes from the airport by train).