I think it really depends on the kind of plugin. If one can say “I run the plugin”, I usually go for a modal, as this implies one action that I can then trigger with a shortcut, select my options and “run” it. When a plugin, e.g., shows information about the scenegraph, however, panels would be my go-to option (as then, having it open while clicking through the scenegraph is essential).
For my two plugins, I’ve kept my modals since they are purely action-based (“Insert placeholder text”, “Export text”, “Search and Replace”, etc.). For my new plugin (Markdown), I support both. I have a modal for “action-based” things, and a panel that allows clicking through the scenegraph and editing the Markdown version of a text layer one selects right then and there.
All in all, I’ve only had one user requesting me switching to panels (which, considering the amount of support requests I get, isn’t much at all). In the end, one should, in my opinion, decide from a UX point of view what’s better, with the following questions being essential to me:
- Is my plugin being constantly open “worth” having other panels closed?
- Is it faster/more intuitive/easier to use my plugin as a panel or a modal?
- Is there any actual benefit to using a panel for the plugin (as modals are easier to develop with fewer exceptions to consider etc.)?
The one thing I find irritating is that most plugins “moved” to panel-based UIs (regardless of the kind of plugin) as soon as it was released, seemingly because it is the new fancy thing . I think no decision is wrong here, as long as one does it for a reason other than everyone doing it (or it seeming more fancy).