Regarding the OP's question, Ive never had Zivkovic myself, but he seems to be consistently a top buy for whoever gets him. The same can't be said about Barbosa though, he's either a beast, becoming perhaps the best attacking player in the game or a bit of a bust. He seems to be more affected by the flexible PA than anyone else I can think of, so scout him thoroughly and if it shows him having PA with little to no con's, than you may have a Barbosa with the higher end of his PA in your save and should buy him for sure.
I have both.
I use a single striker in my tactic and my make a good rotation with them.
I have neymar in my team so I don't use barbosa every game and he still gets me at least 25 goals a season