Short follow up to this ... in the end I did both the liveaboard on Celebes Explorer, and stay on Sipidan. The boat owners also have a resort on the island, and the cost of the liveaboard was only slightly higher.
I can highly recommend it. Did 18 dives from the boat and 12 from the island . The boat dived at Sipidan, Mabul, Kapaili, Mataking, Sibuan, Bohayan and Siamil. Dives were mixed, with poor vis everywhere, but some sites (such as Bohayan) were definitely better than Sipdan for corals and small dish. Sipidan was the only place we saw large numbers of sharks, turtles and barracudas.
The boat is big, cabins and bathrooms big, food was excellent and the crew of 12 were great fun.