Just to check on you, do you mean across the day with 1 use of frenzy? Because an extra attack from frenzy is always better, but if a combat only lasts 3 rounds then that is 2 frenzy chances to 3 Zealot chances. Considering Zealot procs on 1 hit, while Frenzy takes a separate attack roll, Zealot has a higher chance of landing each round (two advantage chances at 5th level and higher).
3 to 2 Zealot to Frenzy is frenzy better 3rd-5th level, equalish 6th-9th, and then Zealot takes off.
Very interesting.