The deciding factor: the length of the day
The 6-hour option links a guided walking visit of Stone Town and the crossing to Prison Island. The 10 to 11-hour one starts with spice farms, then follows the same pattern. The gap is four to five hours, to weigh against the rest of your stay.
What the town offers
Both options go through Stone Town, a UNESCO World Heritage Site, with the House of Wonders, the old fort and the former slave market. The long option also mentions Freddie Mercury's birthplace and the Palace of Wonders.
What the island offers
Prison Island was a slave prison, then a quarantine station for yellow fever. People come mainly to see the giant tortoises, some of them over a century old. The long option adds time for swimming on a reef beach.
What is included or not
The short option does not include entry to the monuments, which costs extra. The long day includes neither meals nor drinks. Check the activity listing before you book.