The four people ahead of me with zeta's in my shard all have a zVader and were giving me a hard time in Arena, I needed them to help me to get Rank 1 (it's a really cooperative shard). They also were able to solo the whole Rancor raid, zVader with fast empire and sith gives enough TM reduction to control the Rancor (put QGJ and Rex in and you can even auto it, at the new 4* speed it's over in 15 minutes...).
But when my zeta was ready I gave it Maul and now I was giving the other guys a hard time, zMaul easily beats zVader in Arena. Now the other guys had to put their next zeta on Maul, and the top Arena is dominated by zMaul now (not only my Arena shard). The sole Rex, Chaze team gives us a hard time though, this is a good defense against zMaul because you can't prevent tenacity up from either Rex or Chirrut.
Conclusion: If you want to reach/stay top Arena ranks, you must zeta Maul, especially if your speed is not top. He doesn't give you much for Rancor or hAAT though, there zVader is the best choice. Arena is most important part of the game, so give the first zeta to Maul, the second to Vader.