ceejay13,
Both rides begin with tremendous G-forces. The start of R & R Coaster is over in a flash and you're off to turns and loops through the night sky of LA. The launch of Mission:SPACE lasts much longer, and the effect of these launch G-forces is like nothing I've ever felt! I scream in both rides, but my scream in Mission:SPACE lasts until I'm out of air to make noise with. Astronauts say that the experience is quite similar to a real shuttle launch. And right after the G-forces of the launch, you feel "weightless" in space! Mission:SPACE is a unique and complex attraction, different from the roller coaster experience. My wife felt nausea and loss of balance after Mission:SPACE and probably won't ride it again, although she felt she had to do it at least once!
Steven Barrett, author of "The Hassle-Free Walt Disney World Vacation" and "Hidden Mickeys, A Field Guide to Walt Disney World's Best Kept Secrets", both books published by The Intrepid Traveler Publishers.