Wicked's book is dull and poorly written, with horrid character devlopment(hence why it lost the Tony to Avenue Q), Wicked's costumes are amazing, Wicked does have the "wow" factor, but there's only so much "Ohyeahhhhhwooooaaaahhhhuhhhhhyeeeahhh" that one can take. Oh and the ending is beyond ridiculous
Hairspray has better character devlopment, and you actually care about what happens to them, the score is more consistent and fun. There is not really a "wow" factor here, but it doesn't need one because it is a better written show, and it can hold it's own without soaring big belty numbers.
You can't really compare the two because they fall in two different categories, Wicked is this "dramatic" piece with bits of comedy thrown in, where as Hairspray is funny throughout. I say see Hairspray, I saw both of them, and I wouldn't see Wicked again, but I would see Hairspray again. But like NJ said, it's all opinion based.
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