That's our opinion after taking the official tour. After you see one of the jail cells, it's pointless to see 10 more identical ones in the hot interior. Besides the jail cells, the torture chamber and a few other rooms that you see, don't deserve 20 minutes total. Doge's Palace secret interiors falls into the category, "since we have time in Venice, let's go on this tour". IMHO.
This is an apples and oranges comparison, but St. Mark's Basilica is truly spectacular.