Downtown Louisville and downtown Lexington are less than an hour and a half apart by interstate. It really depends on what you want to do since both cities are different. Lexington is where the horse farms are and Louisville is where the Derby is held. Louisville is considered more of a midwestern city while Lexington is more southern.
Louisville: has the bat factory, Churchill Downs(I don't think they're running), the Belle of Louisville, Six Flags over Ky. and across the river the casino and the state park about the geology of the river.
Lexington: has the Ky. Horse Park, the Mary Todd Lincoln house, the Gratz park homes, Keeneland(which isn't running either) and the Red Mile plus Ashland and Shakertown at Pleasant Hill(a night here in one of the "Family" houses is special - the food is unreal - the desserts decadent if simple) are just down the road.
Close by is Bourbon Country as is Boonesborough.
Run Bardstown, Versailles(in France, its Ver-sigh; in Ky., its Ver-sales) and Midway through the T/A besides L'ville or Lex for a better idea of both hotels and attractions in this general area. As with most areas, the bulk of the lodgings for both cities are on the beltway or near the airports if you're staying in town.
To answer your question, if I were staying at only one, Lexington, but a night in each is doable.
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