Having the coin in an NGC vs. PCGS holder will make a huge difference. Although NGC and PCGS are widely considered to be the top two grading services, they are not identical. When it comes to market value for PR70 deep cameo coins, PCGS is way ahead. The NGC population for this coin is something like 207 while PCGS has only certified about 16. I saw one Heritage auction lot in a PCGS slab that sold for over $17,000 (!) in 2007. An NGC coin in the same grade went for just $81 back in 2002.
Considering how many 1999-S silver proof sets are out there that haven't been submitted for grading, those population numbers could explode. Top-grade, modern proof coins are a poor place to park thousands of dollars, in my opinion.