I have always used coir, works for me. I don't have a car in which to collect sackloads of anything from GCs, so it's largely a matter of convenience. The bricks take up so little space in storage, can be prepared whenever you want them, are clean to handle, and there are no weedlings to confuse you. I usually buy them from the Organic Gardening Catalogue, they sell nutrient granules specially formulated for use with coir. I also add two handfuls of seaweed meal per brick which is supposed to aid water retention, but I have never subjected that to a randomised controlled trial!
If you want to make your own peat-based mix, the OGC also sells bog-friendly peat, which is recovered from reservoirs.
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If you want to make your own peat-based mix, the OGC also sells bog-friendly peat, which is recovered from reservoirs.