Hi Son of Witz,
I don't think there is a strong difference between them at all.
Mantis came up with a useful (to me anyway) rule of thumb a while back.
"Wolfe uses (or seems to use) "aquastor" and "eidolon" interchangeably.
However, at times in the past when I was trying to ferret out the different forms of "ghosts" involved (i.e., hair-splitting)
I leaned on "aquastor" as being (1) machine generated (as opposed to biological mind generated),
(2) having physical presence;
leaving "eidolons" to be the (1) non-physical, beamed directly into the brain, seen by nobody else forms."
http://www.urth.net/urth/archives/v0009/0739.shtml
To be honest, I think the difference is akin to explaining the difference between a "ghoul" and a "ghost" to someone.
(Nitpickers have at it, I haven't checked my OED so there could be a distinct difference between the two!)