Some Random tips I picked up when reviewing some of the web pages. The basic choices are:

Wood:
Hardwood:
  - IPE and other tropical hardwoods have the longest life 30+ years and greatest environmental impact and highest cost.
  - Black Locust and Sassafras: long lifespan (20-30 yrs), Moderate Cost, Can be oiled annually to keep color rich.
Soft Wood:
  - California redwood Western Red Cedar, Cypress:lowest lifespan (10-20 yrs), highest maintenance .
  - Preasure treated pine - Not recommended

Consumer Reports

Scores 72-53
Score - Type Cost per linear foot
72 LockDry Aluminum $5
64 Western Red Cedar $3
63 Natural lpe $7
61 Envision Evergrain $6
59 Fiaberon Horizon  $6
59 TimberTech AZEK $4
see many more 59-58
56 Trex Transcend $49t 
56 Natural Southern Yellow Pine $1
56 Natural Redwood $4
53 Trex Enhance $2

In most reviews (except Consumer Reports above) Redwood was rated on a par with western red cedar or better.
One review said Cedar is preferred in the South and Redwood in the Northwest, maybe because of availability and price.

The Redwood Guys say
"The Winner is, Redwood. Its close and often boils down to subjective personal preferences, but Redwood does have some clear advantages in durability, cost, and being the environmentally kind option." See: California Redwood Vs Western Red Cedar
Summary: See The Decking Superstore above for details,

WEATHER RESISTANCE
Both have natural compounds that make them resistant to insects, moisture and rot and decay so they both stand up incredibly well to the elements and wont warp or age prematurely.

APPEARANCE:
Redwood is a medium to darker red-hued wood ), and while cedar does come in reddish tones, it can also have a lighter honey-yellow and almost white color as well.
Cedar has less natural color and most people stain it.

Both are available in a variety of different grades, as well, which means you can choose cedar and redwood in a clear, tight vertical grain, with a minimal amount of knots, or in a rustic, knotty look. Its important to note that knots arent a deficiency or a sign of an inferior product. Theyre where the branches were on the tree and are an intrinsic part of the look of the wood. Clear, knot-free wood comes from larger, older trees.

SUSTAINABILITY
Although were talking about differences here, both redwood and western red cedar are sustainable, renewable, and environmentally friendly.

DURABILITY
Both species are considered softwoods, but on the Janka hardness scale redwood is 23% harder than western red cedar.

MAINTENANCE
Both cedar and redwood are pretty similar when it comes to cleaning and finishing

COST:
Prices vary depending on your region and supply, but generally when comparing equal grade to equal grade, redwood tends to be a little pricier, but not by so much

See fiberon vs trex

The deck should be slightly lower than the floor insidce to prevent water damage from splashback from rainfall.

Wood decking requires a gap between the substructure and the ground. Having at least 7 inches of ground clearance is recommended to provide adequate airflow to prevent moisture accumulation, damaging the substructure and fixtures.

You may want to change deck heights either between spaces that join the deck or between deck levels themselves. For example, you may want to use steps immediately outside a back door to drop down to a lower level deck. Consider the configuration and size of your stairs to determine how many steps youll need to have installed to achieve the preferred height. Safety should always be a consideration. Steps can be dangerous and you should consider whether or not stairs right outside your threshold pose a problem. You may also want to consider a handrail if you are descending more than a couple of steps.

A well-constructed deck must have space between boards to allow the wood to drain, dry, expand, and contract without suffering harm. You must also leave the appropriate size gap because too much gap becomes a safety problem.

Deck flashing is critical when it comes to making sure you keep unwanted moisture (in other words, all moisture) away from your home and its foundation. Flashing connects your deck to your home while redirecting water away from it.

For a small space that fits a table for four to six people to sit around, about 150-200 square feet of deck boards is required. This size allows space for guests to relax and move around but wont accommodate much more than a simple sit-down dinner for a few people.
The average deck size is between 300 and 400 square feet. This size provides enough space for around 10 people to have a sit-down meal.

Bpard spacing: For kiln-dried wood with a relatively low moisture content of around 10 to 12% and a relative humidity level of about 35 to 45%, the final deck gap should be about 1/4 inch. However, if the deck installation occurs during drier weather, with lower percentages of moisture and humidity, you may need to add an additional 1/16 of an inch, to account for additional expansion.

Air-dried decking can often be installed with a gap less than 1/4 inch (6.5 mm) because the boards will likely end up shrinking in width rather than expanding. When installing new air-dried decking the proper gap ranges from 1/8 inch (3.5 mm) 1/4 inch (6.5mm)
Soft woods pine, fir, redwood usually have a higher water content so will shrink more.

Deck Board Spacing Guide: How to Properly Space & Stagger Deck Boards decks.com
Gaps that are narrow (1/8), can catch debris, specifically on top of the joists, and can be difficult to clean out. Larger gaps, such as 1/4, allow more of this material to fall through the deck.
For example, a 16-ft pine board could potentially shrink around 1/8 to 3/16 in length, and " in width as it dries.
The 2x6 is going to expand and contract more than a 2x4 because theres more wood or composite decking in play.
For example, a 16-ft pine board could potentially shrink around 1/8 to 3/16 in length, and " in width as it dries.

My notes: My contractor said he can install the boards next to each other, because the gaps will occur when they shrink.
However I needed to remove a board to install some electrical wiring and I couldn't get it up because it expanded when it got wet.

Links:
How to Build a Deck in 10 Steps
WHAT SIZE DECK SHOULD YOU BUILD?
Renovating your backyard? How to decide what size deck is right for you.

Deck Paints and Resurfacers in products