1. Definitely cels!  Sketches are really cool 
for those CG shows, undoubtedly, but nothing 
beats getting in an awesome cel.  In fact, I 
recently paid much more for a post-production 
cel than I ever would for a sketch, because cels 
are just that cool.  Even though the sketch 
would actually be from the show, it's just 
not 'a cel'.

2. Like mentioned by others, it's what's seen on 
screen, that helps my cel choice.  But also it 
is because I feel like not only do I own a 
higher quality piece of art, but also more work 
has to go into a cel.  All the stages of 
sketches, plus painting by hand.

3. When I do buy sketches I prefer genga and 
douga.  Genga especially are just pretty to look 
at, and are easier to imagine in the CG shows.

4. I like to get sketches based on the same 
criteria I have for cels.  Either nice shots of 
characters I like, or pretty cels that inspire 
the feeling of 'art'... or often a combination 
of both.  Key Animators, I've found out, aren't 
that important to me.  I never ask about it, and 
when I get cels/sketches that aren't from Key 
Animators, I simply shrug and move on happy with 
my purchase still.  Of course, I've noticed that 
most sketches/genga seem to be only made by Key 
Animators.  Anyone else notice this?