For decks, you have to consider 2 different components - the structure materials and the decking materials. The structural stuff is what goes underneath and gives your deck its shape, strength and stability. The decking is the superficial material on the surface. They don't really have to be the same. In fact, for really large decks that require high-strength support, people resort to using metallic structural supports (metal studs and beams) but may finish the surface itself with wood or some other decking material.

Aluminum is nice because it's weatherproof, light-weight, durable, easy to clean, and you don't have to worry about pests eating it away. It's great for waterfront decks. For me, the problem is that I haven't really seen an aluminum deck that looks stunning - it typically has a very industrial appearance, even if it's surfaced with some latex-based coating. It's also pretty expensive relative to some other materials. Aluminum is also a metal that is prone to scratches and kinks.

If you're going for wood, you'll need either pressure/chemical-treated lumber (usually fir/pine), redwood, or exotic hardwoods. Untreated lumber will quickly rot away or be eaten by termites and other pests, so avoid that. Redwood is extremely nice because it's reasonably priced, resists mold, and contains natural oils that make it more water-resistant and pest-proof. Not to mention that it has a great-looking grain that goes from pale honey-colored to rose-colored. Redwood often lasts longer out in the open compared to treated softwoods. For treated wood, strong chemical treatment (often some micronized copper compound) is required for the structural parts, especially the posts, but those chemicals are often toxic, and you don't want them on the decking itself (it also interferes with sealants and stains), where you'll probably opt for an oil-based pressure-treated preservative. Whether you use pressure-treated lumber or redwood, you'll need regular maintenance to apply sealers every year or two, and recoat the deck stain (use exterior waterproof stains) every few years as the weather and UV light will degrade the stain (usually, the more opaque the stain, the longer it lasts). The problem with both treated lumber and redwood is that they are soft woods, and can be easily scratched and scuffed.

On the other hand, you have exotic hardwoods, such as teak, ipe, and tigerwood. These boards are absolutely gorgeous and can easily survive 20+ years with very little maintenance. They are super dense - waterproof, insect-proof, and in some cases even fireproof - and they don't scratch easily so you don't have to worry so much about the deck furniture or foot traffic. You should periodically use a hardwood oil-based sealer. Unfortunately, the price for these boards are extravagant and the trees themselves are rare - if you go this route, please ensure you are purchasing materials only from responsibly harvested sources (or recycled sources). Because of the price, they are typically only used for decking, and the structural parts of the deck are usually some other material.

Another choice is composite decking. This is basically recycled wood pulp/fiber/bits molded into a board with weatherproof resins (do not confuse with MDF or particle boards). Depending on the price point, it can look like really cheap molded plastic to pretty convincing wood boards. They are super easy to maintain (just hose them off with water every so often to clean). Unlike aluminum, you don't need any special tools either. Composite decking often comes in kits, which make installation simple (even hidden fasteners and hardware for a clean exterior look). But it's not all rosy... composite decking is prone to sagging, so you usually have to build more structural support. Although some companies have structural composite "lumber", I wouldn't recommend it at all, and would say that if you want to use this stuff, use it for decking only. Composite decking also has different problems depending on the manufacturer - a lot of people complain that some brands quickly got faded spots and flecks after just a year or two of being exposed to UV rays, and some of them also suffer badly from mold problems.

Another option, which can look really good too, and is easy to maintain, is exterior tile or stone "deck tiles". In this case, the structural parts are built using wood, metal studs, or even poured concrete, then you put down a weatherproof underlayment and attach exterior tiles or stone to the surface. The price will vary greatly depending on the type of tile or stone, but they can be super durable and easy to clean. Just apply some sealer every year or two to keep it looking new. These tend to be very heavy though, so large deck areas often require a whole lot of structural support, unless it's a ground-level deck.