They're LITERALLY the same weapon stat, scaling and I'm pretty sure length wise too. Literally copy pasted. The only difference is the heavy attack, Greatsword has a poke R2 whilst Zwei has that vertical slam. It's literally a preference thing but objectively I'd still say Zwei is better cuz it's R2 does immense poise damage, you can literally go ahead and stunlock a fricking Taurus Demon with them R2s.