I think that backing out of the deal is not a good idea, although 
it is a personal matter and takes into account finances, 
possible future dealings with the seller, etc. I don't know 
that an outsider can answer the question very well for you ... 


As for value of the cel in question, unless you are 
thinking of resale why not pick the one that you think looks 
better? If it's resale, some people would be irritated by the 
lack of a sketch, whereas others will be attracted by the key 
aspect and copy background. It's a tough call, and once again, 
depends on the person. 

In your shoes, I'd honour the first 
deal, and count the appearance of the second cel as bad timing. 
It's happened to me before that I've ordered a cel and seen 
cheaper ones become available, but rather than cause ill 
feelings with someone by backing out I'd go through with the 
deal. 

Good luck with your decision!