...

Finally, the conclusion can be drawn that subtrees are better. The daily workflow does not need to be changed. Only if the updates from the external repository shall be included, special commands are necessary. An occasional user of the source code does not need to know anything about the subtree. Additionally, in contrast to submodules, it is possible to adapt the external code to the needs of the host project without changing the code on the external repository.

Links

Subtree Commands for Use-cases