Everyone in my family loves Space Mountain, and it is definitely a must do on our list in the Magic Kingdom. It does tend to be a bit rough at times, and because you can't see what is coming up and it does turn and twist on itself, it is hard to brace yourself for what it coming up.
I would DEFINITELY NOT skip this one. It is a Disney Classic. At least ride it once, and if it is not to your taste, don't go back. I have a feeling, though that you will not get your 10 and 12 year olds to agree to only going once! I know I can't keep my 9 year old off of it!
The seating configuration is that each car has 3 seats that sit one in front of the other, and there are 2 cars joined together. All passengers must sit alone. Try to get in the 1st car, if at all possible, because it is less jerky. Also, there are 2 tracks, and the left side, as you are coming in from the main ride seems to be a bit smoother.
As for it being lighter in there, I haven't noticed any difference. I still can't see what is coming up. It is a really awesome experience!