I faced this decision about my Labor Day ride. The difference between 65 miles and 100 miles is tremendous in terms of exhaustion. I ride 70 -80 miles fairly often but when I rode my one and only century the last 20 miles were hard. If you would like to enjoy the ride, chat it up at the rest stops and finish with energy left, then I would stick with the 65 miler. It is still an amazing accomplishment. (This was my decision on Labor Day). If you goal is to test yourself to your absolute limit, then I would see if you can arrange to at least ride with someone else for the 100 miler. It is very difficult to be in those last miles by yourself.
Whatever you decide, you are an inspiration. I have followed your progress and cheered silently from the sidelines. Good luck!
"Nothing compares to the simple pleasure of a bike ride"~John F. Kennedy