From what I have just read about Webby, it seems like it does not support topics and categories like Jekyll does. However it does have a wide range of rake tasks which do some things like offer a variety of deployment options, logging to growl, and validating links.
There are differences in the philosophies behind the 2 projects. They are both static site generators however Jekyll is more oriented for maintaining blog-style sites. Webby lacks the ability to organise content the way that Jekyll does.
Perhaps most importantly, Jekyll is now available as a free, hosted solution with GitHub - a great way of inviting the large community of developers active in that community to contribute to the project.
I am not saying one is better than the other. They seem to accomplish slightly different things although the method in which they go about it is somewhat similar.