Whether renewing an existing bed, or creating a new landscape area, mulch selection is paramount for successful gardens. Terrain, sunlight exposure, and plant selection will be your guiding factors when deciding to use rock or mulch for your garden beds.
Slope and runoff will determine how much of your installed material will remain in place during rain and wind. Stone is a much better option in high-grade slopes due to resistance to erosion.
Mulch is still a great option in moderate sloping areas, especially with a retaining wall or some type of edging.
You may be “inclined” to automatically associate terrain with drainage. Just having an incline isn’t the whole picture of drainage. Also, many garden beds are built on flat areas.
Mulch does have decent ability to retain and drain water, however stone is a hard material with plenty of space for water movement. If you you have an area that pools water in your garden, you may find your mulch consistently developing fungus.
Both stone & mulch have their pros and cons when it comes to maintenance. Mulch is a loose material that can be blown or raked away more easily during leaf removal.
Stone however does get dirty, moved around, and is more difficult and expensive to replace. Stone can also cause growth and root spreading concerns to specific types of plants.