Installation
With parts that snap together like a child's toy, vinyl railings are easier to install. Depending on the configuration, you can install vinyl railings in a few hours. Aluminum rails, without the snap-together feature, take longer to install because parts are more likely to need to be bolted or screwed together. Powder-coated aluminum railings are typically made in panels that may or may not have to be custom built, and can feasibly take two weeks or longer to acquire. Vinyl rail systems are more available, and you can typically pick them up the same day as stock items at a retail center.
Durability
Shrinkage and expansion can deteriorate vinyl railings after a few years, even under ideal conditions. Vinyl also melts in proximity to flame. The problem is exacerbated by extreme hot or cold weather. Powder-coated aluminum is resistant to weather conditions, will not corrode or rust and shows no significant expansion or contraction even under severe conditions. The consistent shapes, dimensions and fire resistance of powder-coated aluminum railing make it more durable than vinyl.
Light and Heat
Most vinyl railings have an ultraviolet coating that protects it from sun damage, but it's typically not sufficient to prevent vinyl from fading or cracking over time. Also, vinyl is considered a fire hazard, releasing toxic fumes if it burns. Powder-coated aluminum railings are resistant to sunlight and show no adverse effects. One advantage to vinyl not typically noted when comparing these products -- aluminum gets hot in the sun, while vinyl does not.
Options
Vinyl has more design options than powder-coated aluminum. For example, vinyl offers more privacy than aluminum railings due to the availability of add-on panels that you can easily attach. Add-on panels can be added to aluminum, but they detract from the traditional appearance of aluminum rail designs. Vinyl also can be manufactured to look like wood so it matches an existing deck. However, vinyl is available in limited colors while the powder-coating on aluminum forms a durable skin with thousands of available colors that will not fade over time.