Deck Staining

I live in Michigan about 5 miles inland from Lake Michigan our deck is on the south side of the house with very little shade from trees. The deck is new pressure treated pine and is about 6 mo old and has never been stained. My question is which stain should I choose TWP 100 or TWP 1500? I plan on cleaning and brightening the new wood with Restore-A-Deck cleaner and brightener kit. My deck is 168 sq ft w/42 ft of railing 3 feet high supported by railing posts. There are 9 steps 3ft 3in wide and the back of the steps are closed. My next question is how much stain will I need and is it necessary to apply two coats? I was also wondering if it necessary to stain the underside of the deck?