Which islands' finches were less important to Darwin at the time than their mockingbirds?

The Galápagos. The finches are the icon, and they were nearly useless to him at the time: he did not label most of them by island, assumed several were blackbirds, wrens and grosbeaks rather than a single related group, and had to reconstruct the localities afterwards from other people's collections. John Gould identified them as one closely related family, back in London, months after the voyage.

The mockingbirds are what actually moved him. Those he had labelled properly, he noticed they differed from island to island, and it is in the mockingbird note in his ornithological diary that he first writes that such facts would undermine the stability of species. The tortoises were the same story - the vice-governor told him he could tell which island any shell came from - and Darwin had been eating them and throwing the shells overboard.

The finch story hardened into legend much later, largely through a 1947 book. The birds have since more than earned it: Peter and Rosemary Grant spent forty years measuring every finch on one small island and watched beak size shift measurably within a single generation after a drought, which is natural selection observed directly rather than inferred.