Of course it would depend on environment. But Berserker Zero has Arondight to start early. Remember how many lives Caliburn took. It's likely that Arondight will take more. And then there's a world of resources left for Berserker Zero.
And I haven't played Heaven's Feel yet, but Caster's magic ability never impressed me that much. She got her ass beaten by both Archer and Rin (the second one being a surprise of course). Caster Zero, on the other hand, made a giant fortress-sized monster and needed an Excalibur to do him in.
Caster's more of a smart use type of Servant. With the right user, Caster could potentially have a Servant monopoly. By herself though, Caster's one of the weaker servants.
Fate/Zero characters are haXXed...