Hey TT, :)

I would suggest going with the 11 x 14 in the 
case that you obtain oversize cels in the 
future.  Those wont fit in the 9 x 12.  
Also, you can save yourself some money and buy a 
portfolio from an art supply store.  Basically, a 
cel book is a portfolio...in fact, my portfolios 
are made by Itoya, so you're probably looking at 
the same thing.  You'll probably save yourself 
some money on the book itself, and then some more 
on shipping.  Hope this helps! :)

Sean