It will be cheapest to rent in the valley, but if you are staying at the mountain, then renting at the mountain (while costing more) will give you the options of swapping out skis if you wish. High performance skis at Alta and Snowbird generally go out of the shop as "demos" and are more expensive than a standard rental. High performance demo skis will (probably) cost between $35-45 per day. You might be able to negotiate a multiday rate but don't count on it). If you are in the market for new skis, then most shops will subtract all or some of the money you paid for renting from the cost of skis purchased in the same shop. There are several shops for renting at both Alta and Snowbird. Hope this helps.