Both methods are used. I believe that the 's method is more common.
After viewing 3 books: 1. Digest, 2. English grammar in use, and 3. Practical English usage, on possessive apostrophe, in just one book, the last reference, it is written: for older and foreign names ending in s we can use either 's or '. However it claims that using 's is more common.
To be sure I ask you if it is correct to say
1. Socrates' ideas
2. Socrates's ideas
As a matter of fact, my practical rule is: We use 's except when the word is a plural noun ending in s.
My parents' car.
My boss's car.
Is it correct?
The apostophe 's' is used and is pronounced 'Socrateses', like the possessive used after names ending with 's', e.g. Jameses, Rosses, Thomases, etc
I am not a teacher.