It depends. The only thing we can say for sure is premium WordPress themes typically come with a cost, unlike free themes.
When considering a theme, consider the support the developers offer. Some free options come with no-cost support (typically forum-based), while others do not. Most premium themes include technical support, but this is not a hard-and-fast rule.
In general, we find that premium themes offer a bit more — while they cost more, you get better design and more robust support if something goes wrong with your theme.