UPDATE 2: The authors of both also were considering to merge the projects. [3]
I see XBar seems to be more advanced and more popular.
It has these Advanced APIs like URL schema [0].
and also the GUI for plugins compared to Swiftbar
The tech stack of course is also much different as it's written in Go and Swiftbar as the name applies in Swift.
Although I like that SwiftBar has cron syntax support [1]. I didn't find a similar function in XBar.
A comprehensive comparison in one or the other's repo README could be useful
[0]: https://github.com/matryer/xbar#advanced-apis
[1]: https://github.com/swiftbar/SwiftBar#refresh-schedule
[2]: https://github.com/matryer/xbar/issues/607
[3]: https://github.com/matryer/xbar/issues/607#issuecomment-7752...
TL;DR: BitBar stopped working well in Big Sur and the old code base was increasingly hard to update and build.