Don't know about Stratton so I can't compare it directly but, if you are an advanced skier, the vast majority of Sun Valley will not be too steep for you. The pitch is sustained so it may not be a particularly "relaxing" mountain. Virtually all the green runs would be blue at any other mountain so they would probably be the best bet for kicking back. Also, Sun Valley skiers tend to ski faster than average and the snow is often pretty hard packed by western standards. All that being said, it is one of the best mountains in North America and one of my favorite getaways. I was there in mid-January and had a blast even though it had not snowed in weeks.