Either.
In standard American English, you use an apostrophe without an s when making a possessive of a plural noun ending in s. With a singular noun ending in s, you use an apostrophe AND an s.
(E.g.:"I finally managed to struggle out from under the ***succubus's*** suffocating weight."
“I was determined never again to succumb to the ***voices’*** hypnotic power.”)
According to this rule, this would make your second example correct.
HOWEVER, it wouldn’t be English if there weren’t exceptions and, in this case, contentions. Some authorities allow the following exceptions:
–an apostrophe alone when forming the possessive of a proper noun ending in s (this would render your first example correct according to those authorities)
–an apostrophe alone when an added swould make three s sounds in a row.
(E.g. The ***princess’*** world came crashing down around her ears: there would be no surgery.)
Other authorities—chiefly British, IIRC–allow the exception only in the case of proper nouns of antiquity.
(E.g. She awakened screaming, ***Jesus’*** terrible leer etched forever upon her memory.)
This always seemed a bit arbitrary to me, but it makes a certain kind of sense in a culture so horribly afraid of defacing the icons of its forefathers.
I don’t have any of my style books with me to cite specific sources, but I have researched this a bit in my previous job as a copy proofreader, and remember discovering several contradictory citations. I finally developed a style manual for our own to refer to in these cases for consistency’s sake. We went with the exceptions for proper nouns and the third s sound in a row, my rule of thumb being: write the s if you would speak it when saying the word. YMMV.