Vinyl siding is an extruded plastic material. It usually ranges in thickness from .040 - .046. Aluminum siding is a formed material, typically .019" thick. Vinyl siding has a solid color, while aluminum siding has a baked on enamel paint finish.
Vinyl siding is typically more durable than aluminum being that it is over twice the thickness. Vinyl siding does become slightly brittle during the wintertime. Aluminum siding maintains the same properties year round despite the temperature. Vinyl siding also expands and contracts in greater fluctuation than aluminum siding.
As mentioned before, vinyl siding has solid color, meaning even if it is scratched, the color won't change. Aluminum siding has the baked on enamel finish, and if scratched, the paint will be removed and the bare metal below will be visible. However, due to the expansive properties of vinyl, aluminum siding takes paint better if you ever decide to re-paint. It is not recommended to paint vinyl.
Aluminum siding has lost tremendous market share since the release of vinyl siding. Some statistics report as much as 80 percent of all sidings installed are vinyl. Aluminum siding is still very common for trim pieces because of its flexible properties. It is very easy to custom bend an aluminum trim piece. All vinyl has to be extruded or molded into shape.
In most situations it is recommended to install vinyl siding with aluminum trim pieces. Overall vinyl is more durable, has solid color and is cheaper than aluminum.
Vinyl and aluminum siding have their drawbacks. Composite siding does also. Cement fiber siding provides low maintenance, but unlike vinyl, doesn't sacrifice the beauty and character of wood. It does not rot or crack, and it resists damage from rain, hail and flying debris, and the kids' baseballs like vinyl and aluminum. It is flame resistant. Check out concrete/cement siding; grain is very realistic in comparison to wood, and warranties are at least 50 years.