I think that MR is better for smaller projects i.e. ~10devs - it's lower overhead, just commit while you work then write up a description when you push.
I think rebase-CP is better for larger projects like ~100 devs committing to a repo - linear git history and every commit having a clear description+purpose+review is worth the overhead at that point.
So one-off tools and infra and stuff get chucked into gitlab while "the product" is in Gerrit.