I only accept the first style, because I pronounce it "Jameses pen", not "James pen". If there are multiple choices and one more closely resembles the spoken language, I don't see any reason to save a written letter. Otherwise you have "James' pen" being pronounced "Jameses pen", but "the voters' choice" for some reason is not pronounced "the voterses choice". You end up with one specific circumstance where an apostrophe is actually pronounced, and I really don't like that.

I'm also American.