Like most everyone before me, I think it is a 
combination of both. Obviously, we search for 
series that appeal to us on some level...so we 
must be somewhat familiar with what we're 
looking for. However, cels are art, and art 
should be easy on the eyes to those purchasing 
it. Thus, I think aesthetics definitely plays a 
role as well.

My primary focus of cel collecting as of late 
has been the OAV Mezzo Forte, a series I both 
like and think is beautiful, so it's easy for me 
to buy cels from it. However, Mezzo's 
predecessor, Kite, also has beautiful artwork 
and styles, though I despise the movie with all 
heart. Thus, I'm never sure whether to spend 
money on something from Kite, solely because I 
don't like the OAV, even though I love the art. 
Go figure.