Your question regarding decking material is a common one and the choices that you have been given are indeed the 3 intelligent options.
As is the case with most building materials each of these choices has its upside and downside, here is a brief summary of each.
Pressure treated lumber (PT)
This is wood that is treated with chemicals so that it is resistant to both insect damage and decay. It is going to be the least expensive option of those that you are considering, although still not cheap.
As far as green goes, pressure treated lumber is treated with chemicals which is inherently not good.
Alkaline copper quaternary (ACQ) and copper azole (CA) are the two most common chemicals used to treat lumber.
They are considered less toxic than the chromated copper arsenate (CCA) treatment they replaced in 2004, but both contain much higher proportions of copper oxide than CCA and can’t be handled like untreated lumber.
This is really more of a concern to the installer than to the resident of the home, but you might consider other options if indeed children will be crawling around on pressure treated lumber.
Trex is the trade name of a specific brand of composite type deck material and was the first product made from wood flour blended with recycled polyethylene bags. A number of companies now offer decking made of similar ingredients.
Wood-plastic composites have diverted millions of pounds of plastic from landfills, and the decking shows fewer of the defects that typically affect wood.
Composites are not, however, maintenance-free products.
Because they contain cellulose in the form of wood flour, composites can support the growth of mold and should be cleaned regularly.
Colors are likely to fade over time and deck surfaces can become “fuzzy” from wear. Formulations vary among manufacturers, as does the proportion of recycled plastic.
Cost wise composite decks will be more expensive than PT but less expensive than most South American Hardwoods.
The trade name of the most commonly used South American Hardwood is Ipe (pronounced EE-pay) but can also be other varieties such as machiche or mangaris.
These woods are dense, heavy, and highly resistant to damage from insects and moisture.
They fade from a deep reddish brown to silver with exposure to sunlight and can be difficult to stain due to their incredible density.
Ipé should last for many years outside without any chemical treatment, however it can be very difficult to install due to it’s hardness (3,500 Janka hardness, as opposed to 500 for pine).
Because of these issues if you use Ipe you should make sure that you use a contractor who is reputable and works with these woods regularly as there are many unique details that must be understood in order to install it correctly.
The best way to ensure that any wood is green by nature is to insist on Forest Stewardship Council (FSC) certified wood. This is wood that has been certified to have been responsibly harvested without exploiting native resources or labor forces.
It takes a bit of extra work to source and will likely be about 10% more expensive but it is the best way to ensure that your lumber does not come from clear cut forests and maintains the integrity of the forested areas that it comes from.