I can usually handle gravitron type rides but I will say that mission space was a pretty uncomfortable feeling. There is a lot of pressure on your body and I did also look a bit to the side and got dizzy. I can handle any ride, but I will say I just did not like the feeling of mission space. If you get any a weak stomach at all from rides, I wouldn't recommend it.
They are two totally different rides, their only similairty being they both involve space travel.
While ST is a tilt slam twist hile staring at a 2-d screen ride, MS is a smooth and interesting ride with odd physical sensations.. it's almost impossible to explain, but it is nothing like Star Tours.
I can't do the gravitron anymore...heck, I can't even take the teacups anymore! I did MS before they had the two sides and did fine. I had myself very worked up before because I was SO nervous that I would get sick. Right when I started to freak out and think I was not going to make it, the "lift off" was over and I was fine.
If you do what they say to do on MS during the preshow, you will be fine. Just keep your head back and keep looking at the screen during the launch and take it all in. One of the best ride experiences you will ever have.