In terms of Tacoma bull bar fabrication, there are two flavors of choice: 1) less expensive, 2) more expensive. In more in-depth terms, the two choices are mild steel and stainless steel.
The former, also known as basic carbon steel, has a carbon content of between .3-.8%. This keeps it malleable and ductile, thereby making it easy to form and weld whilst maintaining adequate strength. Untreated, this type of bull bar will rust fairly easily, thus any bull bar that is made from mild steel will have a protective coating to help ward off this issue. The majority will simply use the finishing paint, a powder coat, doubling as the protective layer.
The pricier option is stainless steel. 10.5% chromium content by mass, stainless steel is significantly less susceptible to rusting than mild steel. The chromium content in stainless steel creates a microscopic inert barrier at the surface which blocks oxygen molecules from reacting and oxidizing the bull bar. Due to this natural property, stainless steel bull bars are left in their natural finish and are instead just polished to a high gloss. If regularly cleaned and kept free of debris, stainless steel is more or less impervious to corrosion.
In terms of strength, both materials provide more than adequate strength for their intended purpose and are unlikely to break under mild or moderate stress.