"To take an oath by Allaah upon a lie/falsehood is more beloved to me than taking an oath by other than Him upon truthfulness",

Since taking an oath by Allaah is Tawheed (monotheism), eventhough it is false it is still [upon] Tawheed; however it contains the virtue of Tawheed combined with the sin/evil of lying. As for a truthful oath by other than Allaah then it contains the virtue of truth but with the sin/evil of Shirk (polytheism) and Shirk is greater/more worse than the lie and this is from the Fiqh(insight & understanding) of Ibn Mas'ood.