We have sailed both Princess and Celebrity. (and 4 other lines, which do not make the top cut). Our favorite is Princess, with Celebrity a close second. Itinerary comes first (our last cruise was Princess, our next is Celebrity). They are similar in many ways. One better in one way, the other in another. A lot is about personal preferences. However, Celebrity is significantly more expensive, especially in onboard costs. (A favorite drink of mine is $8 on Princess and $12 on Celebrity, for example.)
Now that we have done 36 cruises, my feeling is that Princess is comfortable and upscale, Celebrity is interesting and upscale. Both provide a great vacation. (The other 4? Royal Caribbean is acceptable, Disney is great with grandkids, we have outgrown Carnival, and MSC is "not no, but HELL no".)