TV cels are from a tv series, while OAV cels are 
from Original Animation Video (straight to video 
releases)  Generally, OAVs are shorter and length 
and have a higher quality of animation.  
Generally, it seems to be that whichever was 
released first is more valuable, simply because 
there will come a point where they are harding to 
find than the more recent series.  For Fushigi 
Yuugi, the TV show was released first, so TV cels 
are more expensive than OAV.  Lodoss War OAV was 
released before the TV show, so those cels are 
more valuable.  Fan opinion is probably the 
biggest factor though... the segment that is more 
popular will have more demand, and thus the 
higher prices.  So there really isn't a set rule 
^_^