I buy for both reasons... I will most often be 
attached to a cel from a series I know and 
love, even if its a small or not as high quality 
cel.

however, frequently the reasons that I start 
watching a series are based alot of the time 
on what the series looks like and its character 
designs. so sometimes I'll become so 
ennamored with a character design that I'll 
have to have cels of it, even before I've seen 
the series.
examples are Card Captor Sakura (I love the 
clows), and Tenshi NN (I love Gab, Raphael 
and Miruru)

hopefully when I actually see the series I wont 
change my mind :)