Comparing X (Celebrity) and NCL is like comparing apples and oranges. As your dates are not too flexible I'd search which cruise ships depart from Seattle on 12 - 14 August and then price each cruise that you are interested in, read reviews on Cruise Critic (
www.cruisecritic.com) and decide which option is best for you.
As for X and NCL, I wouldn't consider either to be luxury cruises, both are mainstream cruise lines. I've travelled on X but not on NCL. If you're looking for luxury cruises take a look at Crystal, Silversea, Oceania, Cunard, Regent Seven Seas, Windstar and Azamara. I'm not sure which of these cruise lines offer cruises in Alaska in August, 2011. Obviously, all these brands will cost more than X or RCL.
AFAIK, none of the suites on X or NCL will have two washrooms, and I doubt 4people sharing one washroom would be pratical, especially on days when the ship is in port.
You should consider the hidden costs of cruising, drinks, onboard gratuities, shore excursions, etc can add up quickly.