As mentioned, they pretty much have identical trips, differing only by which islands they may pop into for a quick break stop. During the cooler months, they may be combining the 'swim with the dolphins' with the 'Dolphin 'Discovery/hole in the rock' tours because numbers are lower. The biggest determining factor is the bay conditions: if the swell is too big then they may not allow you overboard for a 'swim' (if that's your intention) because its unsafe to climb back aboard. Also if there are baby dolphins they may not allow you in. The PODs are closely monitored, and the amount of human interation time limited, but overall they do well. Determining factor? Dolphin Discovery (Explore NZ) boats are a bit groovier, and claim to run on green fuel. Fuller's boats are smaller and a (very little) bit older, but they are both fine for purpose, and run on the same schedules. One thing I WOULD suggest: leave it until the last moment to book, so you can gauge sea conditions. And you may avoid disapointment if it's too choppy to go all the way out to the hole in the rock.