web-presentation | Pure Web Components Presentation Framework | Web Framework library

Artifacts should be used to permanently make available any files you may need at the end of a pipeline, for example generated binaries, required files for the next stage of the pipeline, coverage reports or maybe even a disk image. But cache should be used to speed up the build process, for example if you compiling a C/C++ binary it usually takes a long time for the first build but subsequent builds are usually faster because it doesn't start from scratch, so if you were to store the temporary files made by the compiler by using cache, it would speed up the compilation across different pipelines.

So to answer you, you should use artifacts because you seem to need to run composer every pipeline but want to pass on the files to the next job. You do not need to explicitly define dependencies in your gitlab-ci.yml because if not defined each job pulls all the artifacts from all previous jobs. Cache should work but it is unreliable and is better for a config where it makes it better but is not a necessity.