Both go upside down, RnR twice, Space Mountain I can't remember if it's once or twice.
From the front SM was quite a nice ride, but at the back it was pretty rough. I know they have been doing refurbs on SM, but I don't know whether anything rectifies that problem.
On both rides the upside down sections are smooth, they are not particularly high loops either.
Both have fast launches at the beginning, I think RnR's was the faster.
I think it depends on how sturdy you kids are, I've seen a lot of kids on both rides, but they can both be quite a terrifying experience for adults if rollercoaster aren't their thing!