A terribly difficult choice.
Polybius is lauded for his lack of bias and what appears to be a commitment to accuracy, but unfotunately only 5 of his 40 books survive, covering about 80 years. These cover the Punic Wars in depth and are an excellent resource. (Entertaining too)
Livy on the other hand is often knocked for his biases and the reliance on myth and legend, especially as his writing relates to the earliest history of Rome. However, we do need to acknowledge that he certainly had access to sources that we don't even know existed. In spite of Livy's biases, which are actually fairly easy to see, without him, we have virtually no ancient source material on the earlier history of Rome.
While Polybius is a much better 'historian', Livy's work is just too important in my mind. (There are other accounts of the Punic Wars, even if they did use Polybius as a source) And this is coming from someone who loved Polybius, lol.