Since the recipe doesn't call for a souffle pan (which has higher sides), I'd use the 8" pan. Is it about the same depth? If not, then I'd make an aluminum-foil collar to go around the pan to add height & contain the souffle as it rises.
I'd be afraid to use the 9" pan b/c it'll spread the mixture out a lot more, and reduce your baking time, but you (and I) don't know by how much it'll reduce it.
If you're afraid of butter, use cream. ~~ Julia Child
As you cook, you enjoy omniscience about food that no amount of label reading can match. Having retaken control of the meal from the food scientists, you know exactly what is in it. (Unless you start w/cream of mushroom soup, in which case all bets are off.) To reclaim control over one's food, to take it back from industry & science, is no small thing; indeed, in our time, cooking from scratch qualifies as subversive. ~~ Michael Pollan