I couldn't tell you which to try over the other because I am going for the first time to DW in September. Both have gotten great reviews on this board. We are trying them both. We're doing Chef Mickey's for dinner though on arrival day.
Both are wonderful, but I personally prefer Chef Mickey's - a very fun atmosphere (DH loves to wave his napkin around ). Also, if I remember correctly, Mickey was not present at Donald's - I personally enjoy seeing him to start my morning off magically!