Having visited "Derry" about 8 times, I've done many tours. Back when there were SKEMERCons, we always had a bus, and usually one of the group members would point things out as we travelled along. It also helped that the bus driver and his wife were locals and group members, and they both offered their own commentary. One year, Stu from Betts Books was our tour guide/narrator, and we went to many of the places he now includes in his Stephen King Tours of Maine.
Another year, we had the bus, but Brian King (no relation) of the visitors bureau did the narration. This is probably what is known as the Tommyknockers tour (they should change the name.)
This past summer, the SKMB KingKon used the visitor's bureau guides on a rented bus. The tour was good, but the bus driver might as well have been from LA, as he was totally unfamiliar with Bangor and was a very tentative driver.
If you want a comprehensive, and personal tour of Derry and the area, I would not hesitate to go with Stu's Stephen King Tours of Maine.