SCOTUS has weighed in on a very controversial topic, though since it was not the issue of the case, the ruling cannot be said to be definitive.
In both written and spoken English (both British and American usage) there is a growing tendency to omit the "s" after the apostrophe in the genitive form of singular nouns ending in "s". Traditionally this was the practice only in the plural, thus it would be "New Yorkers' preferences are generally for the Democrats" but "Kansas's voting record leans towards the Republicans". One of the largest parks in central London is St James's Park, both written and pronounced with "James's" as a two syllable word.
Disconcertingly, according to Legal Times's Jonathan Starble Justice Souter seems to be on the traditionalist side of this debate while Justice Thomas's preference would be to refer to his own preference as "Justice Thomas' preference".