I think if it's late enough in the game, the answer is remodel Gold->province, buy duchy. I think that maximizes points gained.
If you're expecting to go through another reshuffle and there's still a bunch of provinces left, you might want to gain gold and buy province.
I think those are the two reasonably viable options; both gain you a province, but one loses you a gold and gains a duchy, whereas the other gains a gold. Which one to choose depends, I think, on how late in the game it is and how good my deck is. Wouldn't want to trash one of my only two golds for a duchy if nobody had bought a province yet, would DEFINITELY trash a gold for a duchy if it was the second-to-last province.