You should also look at InnerSea Discoveries and American Safari Cruises. Personally, I think American Safari is the best way to cruise Alaska's Inside Passage since the yachts are so small (Safari Spirit only holds 12 passengers, Safari Quest = 22, and Safari Explorer = 36) and the onboard naturalists are fantastic.
Both of these lines share the same parent company, but InnerSea Discovery is premium while American Safari is top luxury. This spring/summer is InnerSea's inaugural season in Alaska so there are some good deals to be had.
http://www.innerseadiscoveries.com/
http://www.americansafaricruises.com
Embarkation won't be from Seattle in August, but it will be worth the extra flight leg.
If I had to choose between Celebrity and NCL, I would definitely go with Celebrity.
Happy sailing!