Best place for "new skis" meaning 2008, would be heavenly's main shop on the California side. The advantage is that you can switch skis on the mountain. It used to be that you could apply the cost of up to 4 days of rental towards the purchase of any new ski (which would be around 200$ or so at the $50/day demo rate), but I'm not sure what the situation is currently over there.
Other than heavenly, the big ski shop in south lake is called "Sports LTD" (two shops). I haven't rented from them but that would be a place to check out. They seem to have the most skis. Can't take a lift to it, but real close.
There are a bunch of rental places that have demo's, but they mostly are not the latest new model skis; rainbow and powdermountain and others included; if anyone knows for sure a shop which will have 2008 models available for rent, chime in.
Dave's is on the north shore and no where near south lake. Yes, they have decent skis but IMO not worth the drive (1 hour each way, if the road is open at all) to get a rental. If I was demo-ing on the north shore I would start with the Alpine Meadows rental area which, like heavenly's, is at the bottom of the ski lift so you can try out a few different models.