RnRC is faster and has loops, so those who prefer those type of things will find it more thrilling.
I have "old school" coaster taste and love airtime, which is why I find SM more thrilling. By "aggression" I just mean it's not as "smooth" as more modern steel coasters. I like my coasters on the "less smooth" side! (For the "smoothest" ride, ask for the front seat).
The drops aren't that big, maybe the biggest is not more than 15 or 20 feet. It's just that it's in the dark and you can't see them coming, the biggest one is the last one, and in the back seat, they give good air. If you don't like airtime, staple youself tightly with the lapbar and you won't experience it! (the opposite of how I like to ride!

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I didn't mean to worry you or make you nervous about Space Mountain! If you did OK on RnRC, I think you'll be fine!
SPOILER: Here's some POV of the ride with lights on, perhaps that will help you get a feel for if you think you can do it. (the cameras aren't mounted and can get a bit shaky in spots):
http://www.youtube.com/watch?v=qoEd8rGhKEM
http://www.youtube.com/watch?v=rXyTPdOjWlo