Most of the restaurants on mountain will not take reservations. It's really not a big issue but, yes, you may have to wait around or juggle to get a table. Do check Lookout Cabin at Canyons as it MIGHT although I doubt it.
Both locations do have plenty of base area restaurants that may take a res. At Deer Valley look to hotel restaurants at The Montage, St. Regis, Stein Eriksen Lodge and Goldener Hirsch. At Canyons try The Farm at the Grand Summit or the restaurant inside the Grand Summit itself.
Of all the choices I've listed, the terrace at St. Regis at Deer Valley would be the best for the criteria you've listed. Check to see if they would reserve a table on the terrace or if you would have to go inside to J&G Grill, which would also be nice. You also can't beat lunch at Stein's Lodge.