We have been on both ships on Japan cruises, both are older ships. As Bruce mentioned, the Diamond Princess was built in Japan and focuses on the Japanese market, but I would base my decision on the itinerary.
The Celebrity cruise as mentioned has the better ports. If you go to Japan, Kyoto and Tokyo are two places you want to see and, you also get to visit both Hiroshima and Nagasaki (the Atomic Bomb sites). Shimizu will also allow you to visit Hakone (a mountain area near Mt Fuji.). To us, those are the major sites if your time is limited and you are on a cruise.