This can be a hotly debated topic among growers as to what's the best media for growing plants. In this article, I'll weigh in with my opinion on the subject as there are Pros and Cons to both.

"Sphagnum" is a genus of approximately 120 different species of mosses known as "peat moss." Sphagnum and the peat formed from it do not decay readily because of the phenolic compounds embedded in the moss's cell walls. Peat moss can also acidify its surroundings by taking up cations, such as calcium, magnesium, and potassium, and releasing hydrogen ions. Under the right conditions, peat can accumulate to a depth of many yards. These bogs are slowly building and 80% of the peat moss used in the United States comes from Canada. Approximately .02 percent of the 270 million acres (422,000 square miles) of Canadian peat bog are used for peat moss mining. There are some efforts made to restore peat bogs after peat mining. It is debated as to whether the peat bogs can be restored to their pre-mining condition and how long the process takes. Many peat companies claim this to be a sustainable practice, but that is hotly debated topic depending on where you source your information on the subject.

Pros of Peat Moss:

Free of weed seeds, pests and pathogens.

Can absorb up to 20 times its weight in water.

Contains beneficial microorganisms.

Acidic pH (a "pro" in my opinion because you can add highly alkaline amendments to it).

Contains a variety of elements, especially sulfur, which helps with proper terpene expression.

Excellent habitat for beneficial microorganisms.

Harvested in North America, which reduces the fossil fuel impact to get it to the United States.

Holds 10x to 20x its dry weight in water.

Better C:N ratio than coco coir.

Cation exchange capacity (CEC) of 100-200.

Cons of Peat Moss:

Depletes peat bogs, which requires them to be re-built or sustainably harvested.

Naturally hydrophobic, meaning if allowed to dry out it will be slow to accept water.

Needs to be kept evenly moist for optimal plant growth and health.

Requires hydrating before use.

Coco coir is the natural fibrous material found between the hard, internal shell and the outer coat of a coconut. It is treated before use as a growth medium for plants or fungi by soaking in a calcium buffering solution; most coir sold for growing purposes is pre-treated. Once any remaining salts have been leached out of the coir pith, it and the coir bark become suitable substrates for growing plants.

Pros of Coco Coir:

Coir pH usually runs 6 – 6.7.

"Renewable" resource - byproduct of the coconut industry.

Easier to rewet than peat moss, is not hydrophobic.

Usually cheaper than peat moss.

Different reports list coco as having a water capacity ranging from 8x to 30x it's own weight.

Excellent habitat for microorganisms.

Free of weed seeds, pests, and pathogens.

Breaks down slower than peat due to high lignin content.

Cation Exchange Capacity of 40-60.

Cons of Coco Coir:

High salinity unless properly washed.

Quality can vary depending on batch and source of material.

Higher fossil fuel cost to get the coir to the United States from tropical regions.

Does not contain many trace elements.

Does not contain microorganisms.

Traditionally high in sodium and potassium which can lead to calcium or magnesium deficiencies unless properly treated.

Requires hydrating before use.

Increased incidence of nasobronchial allergy among workers in this industry due to the high amount of dust created.

Some coco coir in tropical regions is being sprayed with pesticides like neonicotinoids, one of the pesticides most commonly linked to honey bee death.

As you can see, it's not a black and white decision as to which growing media is superior. Personally, I prefer sphagnum peat moss because of the existing microbiology and the fact the pH is acidic, allowing for highly alkaline amendments like biochar and calcium in the form of ag lime and oyster shell flour. I've also had issues in the past with quality control on coco coir and heard horror stories of variability between batches from the same manufacturer containing high salinity. Regardless of what you choose, you'll most likely want to amend the media with pumice or perlite for aeration as well as some form of nutrients to improve fertility. I like to add earthworm castings or compost as well at a rate of 15-20% of total media to increase organic matter and biological diversity in the mix. If you want to read more about building soils you can checkout the blog post on 7 Important Things When Building a Living Soil.

I hope this article helped you form a more educated opinion on the subject. Happy growing!

Resources:

The Truth About Peat Moss

Does Peat Moss Have A Place In The Ecological Garden?

The Peat Moss Association in Canada