If I understood your question right.... according to the Chicago Manual of Style (online) EITHER is fine. I use CMS for all my grammar questions.
http://www.press.uchicago.edu/Misc/C...q/cmosfaq.html

QUOTE
Q. When indicating possession of a word that ends in s, is it correct to repeat the s after using an apostrophe? For example, which is correct: Dickens' novel or Dickens's novel?


A. Either is correct, though the main CMS recommendation is the latter. Please consult 6.30 (p. 201) in the fourteenth edition, which has a full discussion of these options, including, specifically, the example of Dickens (and the phonetically similar Hopkins and Williams).