Depends on what you are looking for in a city. HEL would be my choice for the stopover. The downtown bus transport is efficient and reasonable value. Your timing might allow a return ferry excursion to Tallinn so you can see how the Finns do the "booze cruise" to acquire as much alcohol as possible at Estonian prices (typically half that charged in Finland).
+1, fully agree, the trip to Tallinn is really worthwhile. You can get the jetfoil from Helsinki central to Tallinn in about an hour, and both cities fully deserve the look around. Get the tram around the old town in Tallinn. Plus AY Lounge in HEL is worth the visit, especially for the spa.