IF the cels look virtually the same, and the face 
is nice and visible, and eyes are open, Id still 
go with the backgrounded one.  Cels with matching 
bgs are very rare, I think most collectors like 
it when a cel has the matching bg.  Key cels are 
a tad easier to find.  I have a lot of cels...and 
a lot of them are key cels.  But I only have two 
cels with matching bg.  One of them you dont see 
a lot of the bg, but still, it counts the same in 
my mind.  Im a bg whore though, so that's just 
me.  Id say pick whichever cel where the 
character looks the best.