As someone currently working in a project where parts are managed with git and others with fossil, I have a strong preference towards git. One major issue with fossil is the non-existent ecosystem. It simply does not integrate well with other frameworks and tools (e.g. Yocto/OE). Migrating to git entirely would be much better for the team and workflow as a whole, but unfortunately the seniors who are using fossil are quite hostile towards git, for mostly political reasons. The irony is that they have big performance issues with fossil, precisely due to the fact that fossil records everything and the push/pull commands sync the entire repo, not just one branch.

Fossil also lacks an equivalent to git submodules, which should be added to the list of features found in git, but not in fossil.