A lot of the queue lines are worth seeing even if you have no intention of riding, so the CM's will always accommodate the guests who don't end up on the ride itself. You can go all the way to the loading area and then simply let the cast members know you'd like to exit.
I suffer from horrid motion sickness and have managed to ride everything at least once for the experience, by a combination of sheer force of will, dramamine, essential oils and SeaBands. The rides that set me off the worst are Rock n' Rollercoaster, Tower of Terror, StarTours, Expedition Everest, the Mad Hatter's tea cups, and Mission:Space (even the green side, forget the orange!).
The best way to conquer these rides included riding them early (before I was overheated and tired) and on a relatively empty stomach.
Space Mountain has been a minor issue only if I'm already off.
Big Thunder has never bothered me.
I would encourage you to take the steps you know from past experience will help you. Stay well hydrated. And don't push yourself to get on it if you really think it might be a problem; feeling sick is not the way to enjoy the happiest place on earth!
good luck!