Both are a form of procedurals. .ASS files I are maybe slightly quicker because they're the native scene description format but if you look at the .ass file when rendering an Alembic procedural you'll see they are very similar.
Personally it depends on your workflow/pipeline. You will find more efficiency with as few steps as possible. If all of your assets are already Alembic you might as well instance those. I have rendered massive environments with both and the only real difference was how I set up the instance container.