It depends what you're looking at for your cruise.
These two cruise lines fall into two different categories. Celebrity is premium and NCL is mass market/main stream. They cater to different demographics. The ships themselves are similar in size. Celebrity has a more refined quieter atmosphere, limited announcements (which is one trait I like about them). You'll find less children on Celebrity, but they do have children's programs. NCL will be a livelier atmosphere and on an average a younger passenger mix than Celebrity.
As for the service and food I feel both cruise lines are good. Personally, I give the edge to Celebrity. But food and service is very subjective on any cruise line, based on people's personal taste.
You may want to talk to a travel agent who specializes in cruises and can help you decide which cruise line matches your life style better.