Most destinations sell tours because the good things are far apart. South Beach is not that. The whole subject is a mile and a half of island, and you can walk from the sand to the bay in fifteen minutes. So the honest question here is not which tour, but whether a tour is the right instrument at all.
Three answers hold up.
The first is interpretation. The Art Deco district looks like a pretty street and is actually an argument about preservation that ran for a decade, with a vocabulary of eyebrows, racing stripes, ziggurat crowns and a colour scheme applied fifty years after the buildings went up. You can photograph it alone. You will not read it alone. A good guide turns a row of façades into a sequence of decisions, and the walks here are among the cheapest structured activities in the whole Miami market.
The second is access. The water off this island is the one part of it you cannot reach on foot, and the departures are concentrated in two places: the marina at the southern end of Alton Road, and the bay side around Sunset Harbour. Everything that floats leaves from one of those two, and what you are buying is a boat, not a route.
The third is local knowledge of the counters. South Beach eating divides sharply between the pavement economy on Ocean Drive and the places one or two blocks back where residents actually eat. A food walk of eight to twelve people is a reasonable way to find the second category in a single afternoon, particularly on Washington Avenue and the numbered cross streets.
What does not hold up is the hop-on bus and the generic city sightseeing product, at least while you are on this island. Both exist to solve a distance problem that South Beach does not have, and both spend a large part of their time on the causeway.
There is also a quick test for whether an operator actually knows this island. A South Beach specialist writes an itinerary in street names and building numbers: Ocean Drive between 8th and 14th, the 1927 warehouse on Washington, Collins Park at 21st. A Miami-wide operator writes "South Beach" as a single stop on a list with Wynwood and Little Havana. Both products exist and both have their uses. Only one of them is going to tell you anything you could not read off a plaque.
There is one structural thing to know before you book anything sold as a Miami tour. Many of them collect passengers by hotel, and the South Beach pickups usually happen first. If you are staying on Collins or Ocean Drive, that regularly means an extra sixty to ninety minutes in a van looping through Mid-Beach and downtown before the tour itself starts, and the same again on the way home. Ask where the tour actually begins and whether a meeting point is offered instead of a pickup. On an island this small, meeting the group yourself is almost always faster.