Start with how much sun the area actually gets
This is the deciding factor more often than anything else, and it is worth measuring rather than guessing. Stand in the area at mid-morning, midday, and late afternoon and note what is shaded.
St. Augustine handles shade better than most warm-season options, so it is common under mature canopy in this area. Zoysia tolerates some shade but generally wants more sun to stay dense.
A yard with a big live oak in the middle is a different question from an open lot, and often the right answer is not the same grass across the whole property.
Traffic and wear
Zoysia is the tougher of the two underfoot. Its density and growth habit handle repeated traffic better, which matters if you have kids, dogs, or a path everyone takes to the back gate.
St. Augustine is coarser and recovers less well from concentrated wear. In a low-traffic front yard that is irrelevant. On a play area it shows.
Neither is a substitute for fixing a genuine traffic pattern. If everyone cuts the same corner, that will wear through any turf and probably wants a path instead.
Water and maintenance
Once established, zoysia is generally the more drought-tolerant of the two and often gets by on less. St. Augustine tends to want more water to stay looking its best through a Texas summer.
Zoysia is slower to establish, which means more patience up front and a stricter watering schedule during establishment. St. Augustine fills in faster.
Mowing differs too. Zoysia's density means a sharper blade and often a lower cut, and letting it get too long makes it harder to bring back.
Disease pressure
Both are susceptible to large patch in fall, but St. Augustine is generally the more affected of the two in this area. If you have had recurring fall disease, that is worth weighing.
Disease is driven far more by watering time, nitrogen, and airflow than by grass choice, so switching grass will not fix a schedule problem. It just changes how forgiving the lawn is.
We wrote about fall disease prevention separately, and the watering guidance there applies to both.
What we do before recommending either one
We walk the property and look at sun exposure through the day, existing irrigation coverage, drainage and low spots, soil condition, and how the space is actually used.
That last one gets skipped constantly. A yard that looks like a lawn on a plan but functions as a dog run needs a different answer than the plan suggests.
Then the practical question: is your irrigation capable of establishing what we lay? Sod fails during establishment more than at any other point, and that comes down to coverage. Because we are licensed irrigators, we check and correct that as part of the job rather than handing you a phone number.