It really comes down to personal preference and what part of the story you are up to. If you are up to the Alabasta arc, I guess you wouldn't care about the removed characters since they would be spoilers. On the same note if you are in the Davy Back Fight arc and beyond, you would want to have all of the remaining characters, stages, etc.
Another factor is your impression of the dub. If you don't like the dub for the dialouge or the voices, you're better off getting the import since the game might not be as enjoyable with the volume off (but that is up to you).
Another factor is the "price-enjoyment" ratio; if you are going to spend money on something, you might as well get your money's worth. Personally I rather pay a few extra dollars to play the original version than to pay to play the watered done version.
As said before it is all about personal preferance. I do not like the US version of the game because it does not appeal to me, a watcher/reader of the original One Piece. After watching 160+ episodes in Japanese, I personally have gotten used to the original voices and don't like the English voices. While the gameplay is good, that alone doesn't make a game enjoyable. A combination of music (which includes voice acting), story, and gameplay is what makes a game good(debatable). If I cannot enjoy the music of the game I cannot enjoy the rest of the game.
Basically I'm trying to say is import the game if can. If you do not care about the voices, buy the US version. The gameplay is good, although the game would have been better if they included the original voices.