Mission Space actually spins. Star Tours does not. HUGE difference.
Motion sickness generally = a difference between what you eyes are telling you, and what your brain is telling you.
On Mission space, closing your eyes is a bad idea, it will actually make you more aware of the spinning. the idea that you are NOT spinning comes from the screen.
On Star Tours, there is no spinning, period. Beyond that, nearly all the 'motion' is virtual. It happens on the screen in front of you. Close your eyes and all you will feel is a tipping motion, like riding a bike uphill/downhill.
So basically, if you close your eyes on Star Tours, you can pretty much eliminate most of the motion you are feeling.
Harry Potter is a bit like Star Tours, but with a bit more actual motion. Still, the same idea- closing your eyes will greatly reduce the amount of motion you feel, at least if you close your eye for the video portions of the ride. (and maybe open them for the actual motion parts, even though the actual motion is pretty tame. HP doesn't exactly spin....but your seat rotates on the end of a large arm in various sections.