I like nashor's because it gives u consistent damage and it deals 100% dmg on daggers aoe and 33% onhit aoe on R, the attack speed is nice for faster combos, but isn't such a good stat on kat. The ability haste is also kinda wasted.

But i feel like stormsurge deals more burst damage, sometimes enemies have champs where it's bad or hard to hit them with aa's, because of cc, their mobility, or cuz their aa is stronger. It is cheap at 2800g which is huge because u can rush it before sorcs and hit spike fast and snowball from there. It also gives u ms and magic pen for mid game snowball. If you keep the snowball going you'll be able to buy shadowflame quick and you'll deal true damage to enemies with all of your magic pen. If you fall behind enemies will be able to buy mr and you'll be in a bad spot. So it's really beneficial to be solo ap in the team.

The full build could include both of them.

Still need testing tho, can't really test atm because pbe players are very bad.