posted on Apr, 29 2009 @ 07:57 PM
I can't say I 100% believe in either, but I'll play along
To me, the only similarity I can think of is the unrestricted movement.
Ghosts have been said to be able to move through solid objects and simply disappear. They don't seem to be solid matter.
'Aliens', on the other hand, are said to look solid, yet a lot of abductee reports mention them simply walking through walls, doors and windows.
So both can become ethereal.
Apart from that, I doubt there are many other common elements.