Well, there are three types of aluminum wheels.
Cast, milled and forged. Weakest to strongest. Also happens to be the cheapest to most expensive.
Aluminum's are typically lighter then the same size steel wheel. Cast is the weakest but also the most common. Basically they pour molten aluminum into a cast then clean it up once it cools.
A milled wheel is cut on a CNC machine from a solid chunk of aluminum. These are stronger then Cast wheels because there is less likely to have any impurieties or bibbles in the material.
The last is the forged wheel. These are made from discs or billets of aluminum that are hammered until they look like wheels. These are the most expensive because they require the most work. The hammering does make them very strong because of the compression of the aluminum.
Steel wheels are pretty strong but are also heavy. They are cheaper and probably the best bang for the buck. With small tires, it migth not be a big concern. When you move up to 33's or larger, you'll start noticing the weight.
Some other things to take into consideration are the tire. A bias ply tire will weight more then a radial because of the thicker layers. The heaviest setup would be a Bias ply tire like a swamper on a steel wheel. The lightest on the other hand would be a radial on a aluminum wheel.
Which is better? Hard to say anymore. I've been cast and forged used on everything up to a buggie. Really, it's up to you. Aluminum cost more and don't look as nice with lots of rock rash.
You might want to search for Mozman68's thread when he was looking at wheels. It talks about backspacing. I think hsi Jeep look good with the wheels he chose.
On a side note, I run aluminum with 3.75" BS, I really wish I went with more BS.
