For me, a sketch doesn't really tip the balance
all that much--it's more about the cel.  In 
mainstream American animation, sketches are more,
I don't know, collectible?  Especially with the 
older shows like Peanuts, Woody Woodpecker or 
anything by Disney.  Sometimes you can find
sketches priced as much as cels. :)  In anime, 
tho, I tend to view the sketch as a nice bonus,
not a requirement.

Jen