Having built a few decks, let me offer the following:
- Screws are always preferable to nails.....and stainless steel Robertson Head are the best.
- Best deck material would be an imported tropical hardwood such as ironwood or cypress or Teak. However, unless money is not an object, pressure treated #1 yellow pine is a good alternative as is Trexx, a synthetic wood fiber and plastics product which will not crack, split, expand/shrink or warp....but holes must be predrilled. Redwood or Cedar are ok, but they are expensive and very soft, thus prone to marking/denting etc.
- If you are in a snow zone, I'd go with steps without risers (the backs). Makes it a lot easier to shovel them off in the winter.
- Whether to skirt the deck in lattice is a matter of taste. I'd take a look at decks the contractor has done before to see how it looks and be sure to check quality of work.