Can't believe anyone would ask such an obvious question… 😉
8761 is made from "a polyurethane elastomer that resists punctures, tearing, abrasion and erosion", whereas 8663 is made from "an exceptionally tough abrasion resistant polyurethane elastomer that resists punctures, tearing, abrasion and erosion". 8663 is 0.46mm thick, whereas 8761 is only 0.36mm thick. This makes 8663 11.7 g/m2 heavier. 8663 also has a higher tensile stress at 2049 N/100mm vs. 1400, although the strains to failure are the same. The bare aluminium peel adhesion is also somewhat better.
8663 might therefore provide marginally better protection for your bike. However, this does come with a serious weight penalty of around a gram or two, were you to cover the whole bike in it. I suggest you think hard and choose carefully, depending on whether it's more of an XC or all-mountain machine.