Consider your maintenance preferences
If you want a ground cover that needs refreshing less often, rock is generally lower-upkeep. If you want the soil benefits organic material provides, mulch may be the better fit.
Think about what's planted in the bed
Heat-sensitive plants may do better with mulch nearby, since rock can hold more heat. Beds with hardier or more drought-tolerant plants often pair well with rock.
Factor in the look you want
Rock tends to read as clean and modern, while mulch often looks more natural next to dense planting. Neither is right or wrong — it's about what fits your home and preferences.
You don't have to choose just one
Many landscapes use rock in some areas and mulch in others, based on what each specific bed needs.