They don't give "flexibility" ratings for the thinset or grout, just that acrylic additives make it "more" flexible. They do however publish strength ratings which is an indication of the quality and flexibility of the product. Lower strength would indicate that the product is more brittle and more likely to crack. Both prodcuts I was comparing had acrylic additives, just in different forms.
On another note, I found that Custom has a webiste http://www.custombuildingproducts.com/ and a handy product selector. They suggest the Marble & Grantie Mortar Mix for my application which is what I was leaning toward anyway. Thanks for the tips!