Ultimately, it really depends on what you intend to do with them.
* If looking to flip them for profit, buy only the ones that you can make money on.
* If buying for your personal collection, buy whatever makes sense for your collection as the price is approximately what mercury dimes seem to sell for on ebay right now in small lots.
* If buying just to acquire silver, you would be paying above melt so this may not be the deal for you.
Regardless of your reason, I would pick out any with significant numismatic value above silver value whether that is due to better dates or better condition.