What mulch does well
Mulch helps retain soil moisture, suppresses weeds, and breaks down over time, adding organic matter back into the soil. It tends to look natural next to a wide range of plants and is a common choice for beds with heavier planting.
What rock does well
Decorative rock doesn't break down, so it generally needs refreshing less often than mulch. It can also hold up well in low-water or xeriscape-style beds, and gives a clean, defined look.
Considerations for North Texas specifically
In our hot Texas summers, rock can hold more heat than mulch, which is worth considering near heat-sensitive plants. Mulch can help moderate soil temperature and moisture during stretches of extreme heat.
Both materials work in DFW landscapes — the decision usually comes down to the look you want, your maintenance preferences, and what's planted in the bed.