Kizuna does feel softer then its equivalent counter part. z63X is very similar to exbolt63, more durable I find. But z63 is softer and easier to play and I get slightly better power. I would string it the same just to see and you can adjust the next time.
Can someone tell me if Kizuna still produces Z66 Lusty string or if it has been discontinued? The string was still available at the e78 shop, but is now sold out (set and reel).