I don't have a very strong opinion about this, but this is the parent task of T330797: Uninstall Differential (Phabricator application).
I will try to sum-up. Note that this table tries to sum-up thinking about "the thought of the first developer newcomer, you can meet on a random street":
| What | Where is it adopted now | Main adoption reason... | But... | Consensus notes |
|---|---|---|---|---|
| Gerrit | mostly in use by core MediaWiki components | historical reasons | approval processes for repo creation/changes; no "pull requests" | keep Gerrit but "not mandatory" to host new community projects |
| GitHub | mostly in use for read-only mirrors | some users expect stuff on GitHub too | proprietary | keep GitHub but "avoid as primary tool" |
| Differential | in use by 2/3 Phabricator lovers | it's 100% integrated with Phab | no "pull requests" | T330797: don't promote Differential / kill it |
| GitLab | mostly in use by community | it allows massive community adoption | it needs more integration with Phabricator to avoid dispersion | consultation: let's try GitLab and eventually promote to the community |
| Bitbucket™ | mostly in use by important community tools | adopted when there was no GitLab/Diffusion etc. | proprietary; totally separated from both Phabricator and GitLab; not controlled by WMF | T330846: no known discussion |