The bggest difference has already been covered by some, but for me it comes down to what is most important in those classes.
BREEDING- Most important features are soundness, volume, and femininity...if she is good enough to compete with them all in those areas and has the extra thickness, then I would probably still run her breeding personally.
MARKET- The most important here would be product and finish. If she has enough thickness, top and butt to compete with the markets and you are willing to feed her to finish her for the show, yet she gives up a little volume, structure and style, then I would probably run her Market.
From the pictures posted, I wouldnt say this heifer really jumps either way for me at this point. What is her age? I would say for me she could kinda both work ways. I dont see anything that says she is going to go blow the competition out of the water in either class so that doesnt help me make the decision. For me I guess it would come down to the shows I was going to and their rules and what my plans are for the heifer. If you are considering putting her in your herd then you definately dont want to feed her to be finished for a market show. So that makes me lean towards breeding. Also if you are considering putting her in the herd, then you really want that heifer bred before the time she would get to show at market weight, some shows have rules that market heifers have to be open. That again would help make my decision. I would saythat if this is a heifer you want to keep I would personally probably swing more in line with running her as a breeding heifer, especially if they had rules that she has to be open. If they dont make you leave them open, and you want to keep her, then if you think she has enough product to compete with the market heifers without having market finish, then I may still run her market. Its a tough call to make, a lot of factors to consider.