Kzarka
Kzarka has more accuracy(18) then Offin, 1 more casting speed, and 2 less crit than Offin.
Offin Tett
Offin has 18 less accuracy then Kzarka and 1 less casting speed. But with 3 more AP and 2 more crit than Kzarka.
Below 245 AP? Kzarka is better than Offin.
Between 245-269 AP? Offin gives more value for the money than Kzarka.
Above 269 AP? Both Kzarka and Offin are viable, most choose Kzarka because people play with evasion builds/get PEN armor.
Summary
Under 245 AP with Offin (242 with kzarka), go kzarka.
Between 245 AP with Offin and 268 AP with Kzarka (271 with Offin), go Offin.
At or above 269 AP with Kzarka? It depends. Offin against non-evasion stacked opponents. Kzarka against evasion meme builds or geared evasion hybrids. Personally, I think Kzarka is generally best when very geared (arguable).