For cream stouts, you usually don't want much more from your hops than a clean bitterness that balances the maltiness. I often use organic Pacific Gem hops in my cream stouts, a pretty high alpha acid hop without a strong flavor.
Of the two varieties of hops you have, I think I'd go with the Centennial - less citrusy than the Cascade, perhaps. When you substitute, you'll want to take into account the alpha acid of the hops so that you get the right IBUs in your beer. You can figure this out using brewing software like Beersmith, or you can ask someone here, if you need help.
Good luck with your cream stout - one of my favorite beers to brew and drink. http://www.singingboysbrewing.com/sweet-stout.html