I've spotted at Sea-Tac many times. There used to be a designated spotting area across from the airport, but it was kinda hard to find, but its closed now anyway due to construction of a third runway.
Anyway, as for hotels, I would recommend the Double Tree, its the tallest in the area, and the lounge on the top floor gives a great view of the airport.
As for other areas to spot, If the planes are landing on runway's 34L and 34R, the south end of the airport, go to Tyee Golf Course is a great place, sometimes the workers there will give you trouble for being there and not golfing, but usually they let you stay.
If the planes are landing on runway's 16L and 16R, the north side of the airport, there is a perimeter road that can be found without too much trouble, I can't give you real specific directions on how to get there, because my had has always been driving, but its not too hard to find, just head for the north end of the airport, there is even a big turn out area for you to park.
hope that helps you, get ready to photograph lots of Alaska 737s, Southwest 737s, and Horizon Dash 8s!
I'm at sea-tac a lot, so here are some times when the wide bodys arrive and depart.
SAS A340, arrives around 10:00 or 11:00am, leaves at approx. 7:15pm
British Airways 747, arrives approx 2:30pm, Leaves approx 6:30pm
United 777 - there are 3 of those there every day, not sure when they arrive though, but one leaves around 2:30pm
American 777 - arrives ?, departs around 2:30pm
Eva Air 747 - Arrives around 10:00 or 11:00am, not sure when it leaves
Northwest 747 - arrives ?, departs around 2:30pm
Hope I helped you out!