If British and American spellings of a word differ, which one should I prefer when playing Scrabble?
In my experience, the choice depends entirely on which spelling produces the largest score.  I do occasionally play with people for whom the score is less important than the strife that can be stirred up, but even there, unexpected alternative spellings are rarely an important consideration.  Blocking a desirable word is far more satisfying to the player for whom the annoyance of his fellows is the reason to play the game.
So, which spelling should you prefer?  I will go on record as saying that, if you play against me, the spelling you should use is that which gives me the best chance of a high-scoring riposte.  Presumably that would also apply if you are playing against your relationship partner or someone you find desirable and who is very competitive.  Otherwise, you should play as your conscience and your sense of self-respect dictate, and to the devil with the preferences or traditions of those whom you would trample into the dust of the board.