Mini golf at WDW is a fairly common activity for my family in the summer. Our seasonal passes are blacked out in the summer but we still frequently end up in WDW to visit friends and watch the fireworks. Mini golf has become our default way to spend the afternoon. Fantasia and Winter Summerland are two sides of the same coin. Fantasia is a very beautiful, almost professional putting green. Winter Summerland is zany mini golf with a Disney flair. Personally, I like Winter Summerland the best. To me, mini golf is about getting a golf ball into a windmill and not a serious display of a short game.
It does not take a tremendous amount of time to do either course. If you have an afternoon to kill, you can not do both.