You are just starting your Mercury dime collection right? Don't you need the more common coins or do you have them already? The key date is just that a key to the set. Why don't you just buy some nicer common coins first and when you can afford a key, pick the best you can afford?
I always have a problem with the 'best I can accord' bit. If I wait a certain amount of time, I can afford a higher grade. Rather, I determine what top grade I would like in my set for the key dates, and either wait and buy that grade, or buy a few grades down and buy/sell my way up to it. Just one man's opinion.
The coin was AG, and as such the mintmark was weak. It was about half missing I would say, and the dealer didn't want to pay 16d money on something customers would be unsure of potentially. When SO MUCH money is riding on that tiny little D, most collectors wish to know for a fact it is a D.
Ok here is were I differ in opinion from the rest of the guys here. If it is a "hole filler" coin, I choose the one I like the most (in the price range) In the case of a hole filler a shiny cleaned coin usualy wins for me as the dirty/worn ag would reduce the eye appeal of the whole album. However if you do intend to "flip" the coin when you replace it the problem free coin is almost always easier to sell off. In the end it is your collection and your opinion that counts, your doing this for your pleasure so buy the coin that will give you the most.
I would say the AG example is the better buy. Easier to sell a non-problematic coin than one that is original. The grade may be a Good/Details, but the net grade might be an AG or Fair. Collectors of key dates are pretty picky.