Vinyl by itself is not a good insulator, however if installing over decent siding with at least 1/2 to 3/4 inch backer insulation is good. Stay away from the darker vinyls as they may tend to warp and buckle over a period of time. A good grade vinyl with proper insulation will last 40 years.
Aluminum will chalk over a period of time and will need painting.
Most of your energy savings does not come from your siding, but will be through your windows. Upgrading to more energy efficient windows will save you more in heat and AC than either diding will.