What is the practical difference between cement board and greenboard?
Greenboard is water resistant, but should not be used in areas that are going to stay damp, such as tub surrounds.

Once water gets into it (especially if it's not sealed correctly, and there's not the requisite gap between the tub and the wallboard), it will slowly deteriorate and/or mold -- it's water resistant, not water proof.

Yes, cement board is a pain to work with, it's better than having to tear out the whole wall because something was off in the installation of the greenboard.

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All that being said, I believe I've seen some home improvement shows that used greenboard and then waterproof membrane over everything to add an extra layer of water protection when they were doing a shower install (tiling the walls and floor; and there was no tub they were joining up to).

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You might also want to check with your local government -- it's possible that there might be codes that dictate what you're allowed (or required) to use in shower/tub areas.