I've had both on my sled.
Simmons are a better ski for trails, hard packed snow. Almost no darting, smooth ride due to the flex of the ski.
Yamaha, Powder Pros (very similiar ski), perform better in deep snow, and powder. Also to my suprise, they hold a side hill better then the Simmons.
Both are good skis, and are far better then the stock skis. Just depends on the type of riding you do. I do 90% off trail/mountian, so I kept the Yamaha skis, and sold the Simmons.