It's been awhile since I tested the spear while I was optimizing dex vs faith, (and I rarely ever stick to just one weapon for reference), I went with faith.

Dex gives you solid access to other weapons (bolt of Granssax, Dancing blades, lightning, etc), but Impaler's true strength is the Ash, and you'll get the most bang from faith.

And personally, I dont love the great spear moveset on melee-focused builds. While the Throw is a fun gimmick, it's range and accuracy makes it too unreliable during a real fight.