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Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 100,413
Mainz in a pleasant small city. Unless you want very late nightlife or large/high category hotels, stay in Mainz. Something else to consider is how late the train service runs in the evening if you would be expected to join colleagues in Mainz for dinner.
Depending on whether you want IC stays or nights, it could make sense to split your stay with a few days in each place.