This comes down to what you intend to do with the homemade seitan once it's cooked. Simmering in stock will allow the seitan to pick up the flavors from the stock but also yields a juicer, slightly fluffier (but not spongey) seitan chicken. In comparison, steamed seitan will be slighter denser in the middle, and won’t contain as much liquid - making it the preferred choice when you want a crispier sear (like pan-frying/ stir-fries/grilling, etc.). Steaming is also usually slightly quicker.