On the outside, I can't tell any difference, other than one will write upside down (which comes in handy when jotting down notes in the middle of the night) and the other will not. The space pen I use is about the same size as the normal pens I use, and the refills look the same. Obviously inside there are major differences.
As far as pencils go, they work fine but you have to press harder, and they aren't permanent. They also don't give as clear and consistent lines as pens do.