What Gauge (thickness) Material Do I Need?
Aluminum comes in .012, .032, .063 & .080 gauge.
Plastic comes in .024, .050, .070 & .110 gauge.
The .012 aluminum & .024 plastic are not self supporting so they will need to have some kind of a backer behind them such as a tree or board. The .024 plastic is roughly the thickness of a credit card while the .012 aluminum is roughly the thickness of a soda can.
A typical rule of thumb is that the larger the sign the thicker the material, unless the sign is going to be supported by a building ect. Plastic in particular is affected by heat, so if a plastic sign is going to be in a lot of heat and direct sunlight the lighter gauges will start to curl. The .110 gauge does this less then any of the others.
To give you an idea of thickness a standard stop sign is made out of .080 aluminum. This will be thicker than most people need unless the sign is very large or needs to meet MUTCD standards. The .110 plastic is a little less than 1/8" thick.
It may go without saying but as gauge increases so does price.
In regards to fading the gauge will not have an impact.