When I was choosing my material for the bench tops, I found Poplar, Aspen and Cedar to all be acceptable in a sauna hot room. I ended up buying Aspen simply because it seemed cheaper and was in stock at the store as opposed to Cedar and Poplar. The boards were grade "A" without knots and polished smooth which seemed perfect at the time.
The smooth wood is like glass and seems to really heat up. In comparison, when putting up the cedar on the walls, we had a choice of using the smooth side of the boards or the rough side and I chose the rough side. In my mind, the rough side acts like fins on an engine and cools the wood. This polished Aspen I now have on the bench tops is smooth with no fins and gets pretty darn hot. I may try roughening it up, but you think about this afterthought when you select your bench-top wood.