Conceptually, it would seem that half-truths are better than lies insofar as they are closer to the truth. This is especially true for half-truths where the false or deceptive part qualifies as a “white” lie (e.g., “I’ve never said one bad word about your mother to anyone” when you’ve only written bad things about her in your private journal). But in practice, I think it really depends on context. A half-truth might be more likely to successfully deceive someone insofar as it contains, by definition, an element of truth. And a convincing half-truth can be much worse than an obvious lie.
Suppose, for example, that you want to buy a gift for a mutual acquaintance and ask me what their favorite food is. If I say “they like anything with arsenic in it,” you’re probably going to know that I’m lying and trying to get you to harm the person. But if I say “they like anything with chocolate and peanut butter,” you’re more likely to believe me and not consider the possibility that someone can like something while simultaneously being deathly allergic to it. So in that case, the seemingly innocent half-truth is worse—and more dangerous—than the blatantly malicious lie.
These might be silly examples, but they at least demonstrate the possibility of half-truths being worse than lies (even if they aren’t always worse than lies).