A real, triple layer fence unlike troops can't be moved tomorrow depending on the political winds, is cheaper in the long run, acts as an effective barrier to add to border patrol defenses, and sends a strong signal that we mean business.
Fences can be dug under, cut through, crashed into, blown up, you name it, they were made to be defeated. Illegals will be more afraid of armed troops than they will of any fence. Troops will also have the ability to search incoming vehicles.