The stock bumper is designed to absorb energy is small impacts and protect the frame and crumple zones, and to distribute the force across the frame in bigger impacts.
Here is what you could expect with a stock vs. aftermarket (steel) bumper:
1) Low-speed impact (below 5 mph) - stock bumper will absorb the energy 100%. There may be some visible damage to the bumper depending on the direction of the force and nature of the object you hit. Any necessary repairs will be modest. With an aftermarket bumper most likely there won't be any damage.
2) Moderate speed impact (5 to 20 mph). The stock bumper will absorb most of the energy, but there will be some damage to the front grill, hood, etc. Most likely there will be no damage to the frame, so the repair costs will be manageable. On the other hand, with an aftermarket bumper that does not absorb energy very well, it is VERY likely that there will be extensive damage to the frame, and the cost of repairs will be much higher. The energy HAS to go somewhere and it will be dissipated by the weakest link in the chain.
3) High speed impact - there probably won't be much difference, as long as the aftermarket bumper is designed properly.
The function of the bumper is two-fold:
a) to absorb energy in a small impact
b) to distribute the energy to the frame in a high speed impact.
The aspect of distributing the energy is a very important one. The purpose is to use all available structural strength of the car to uniformly absorb and dissipate the energy of the impact. It takes many hours of engineering, including some high level of dynamic FEA simulation to get it right. I doubt that aftermarket manufacturers have the time, money and engineering skills to perform such analysis.
As a side note, this is what makes big difference in crash test rating between European and American cars. The standard NHTSA test is a full-frontal test in which the car is crashed into a perpendicular concrete wall. While this creates a huge deceleration, it makes it "easy" on the designers of the bumper, because the force is by default uniformly distributed over the entire frontal area of the vehicle. On the other hand, the European test (and the one performed by IIHS) is an offset test, that covers only 40% the vehicle's front. This makes the design of the bumper and front-end of the car much more critical. Traditionally American-designed vehicles would score well in the NHTSA test, but would do poorly in the Euro test. On the other hand, any vehicle that score well in the Euro test, will easily score well in the American test. Japan uses both tests to assess their vehicles.
Net, if what you care about is small dings and repairs when off-roading, an aftermarket bumper is a good choice. But if safety is your priority, you are probably better of with the stock bumper.
