Ooo, that's a toughy!  I usually airm for BOTH.  
#1 on list is sentimental value.  If you aren't
familiar with the series and don't LOVE what you
have, then chances are you will not be happy with
it later.
I try to look for both.  I am so darn picky with
buying a cel that it has to be absolutely perfect
to where I KNOW I will never ever want another one
of that character.  This is why I would much
rather save my money and pay $800 for the
"perfect" cel rather than buying 10 of the lower
quality cels.  
I have at the most 2 or 3 cels of my absolute
favorite characters (Inuyasha, S