I wasn't really a fan of these two and I did try both out but I ended up picking Serperior this time.
The reasons why? It has better stats than Zebstrika, it doesn't die too quickly (Zebstrika's defenses are pitifully weak) and it has great movepool that abuses its best stats, unlike Zebstrika who severely lacks physical STAB outside of Spark/Wild Charge.
And trust me, if I could ever get a Contrary Serperior I would Leaf Storm all my opponents to death. Zebstrika's Sap Sipper only provides an immunity to Grass, which isn't all much. I love both designs but Serperior's won it for me.
Sorry Zebstrika, I had to pick the green snake for the above reasons.