The numbers refer to the amount of alchohol by volume, so the 91% is much more "pure".
You may already know this, but ISO is not a universal reducer or cleaner for acrylic paints. Chemistry varies from brand to brand. I've only used a few of them, so I'm not your best source of info on that. This month's FSM has an article on acrylic paints with a good reference chart that breaks down the best choices for most popular brands.
Hope this helps!
Mike