Quality

Many people have the belief that the more something costs, the higher quality it is. For software, this is rarely the case. Paid Minecraft plugins are, in most cases, not higher quality than free plugins. The best indicator for quality in Minecraft plugins is whether the creators are established developers in the community, or whether it's a well-established project.

While experienced developers make high-quality premium plugins, there are also many quick "cash grab" plugins. It can be hard to tell the difference between these at face value, but cash grab plugins are generally much lower quality. If a developer has made a large number of premium plugins, their plugins are likely cash grabs.

Support

When it comes to paid plugins, it can be challenging to get help or get what you need out of the plugin. Many premium plugins have insufficient support channels, and people in community-oriented support channels will be less likely to help with paid plugins.

Paid plugins are also generally not supported by other plugins, as most developers will not pay for plugins to interface with them. Due to this, you may get stuck using more paid plugins from the same author to get the required functionality.

Once you've used a plugin for some time, switching to another can be difficult. With open-source plugins, it's common for another developer to take over when one stops work on the plugin. With premium plugins, not only is that difficult, but it's also often against the terms of the license. Nearly all premium plugins explicitly forbid this from happening. If the developer loses interest, you need to find a new plugin and hope you can easily migrate.

Security

When it comes to security, open-source plugins are often better. While it doesn't guarantee that the plugin is safe, it means that people can freely look over the code. While both free and premium plugins can be open source, it's much more common for free plugins.

Premium plugins often make use of a technique called obfuscation that makes it harder to read the code; this means it's much harder to determine if a plugin is acting maliciously or not. Many private anti-cheat plugins have contained backdoor vulnerabilities in the past, which would be significantly harder to hide in an open-source unobfuscated plugin.

Another aspect of security is how many people use it. Software with more users is more likely to have security vulnerabilities found and fixed sooner. As premium plugins have a smaller market share due to entry barriers, they are likely to have fewer users. On the flip side, however, fewer users mean fewer people will try to exploit them. This fact is irrelevant for a targetted attack, however.

Large server networks are generally a standard method of discovering security vulnerabilities, as they often pay developers to look over the open-source plugins they use. As these networks are less likely to use premium plugins due to having paid developers, they won't find these issues in paid plugins.

Overall, free or premium plugins aren't guaranteed to be more or less secure, but premium plugins can carry a higher risk.