Really, for all epiphytes, there is no 'preferred medium.' They would grow on a treebranch with no media at all, and I have seen amazing displays of phals where they hang from the ceiling on a cord with the roots radiating out like Medusa on a bad hair day.
Put simply, media is something we force on our plants to make our lives easier.
Thick rooted epiphytes (Like Phals and Cattleyas) are better off in very coarse bark (2") that has been sieved to remove all the small bits.
Fine rooted ones like dens, oncidiums etc are best in fine bark that again has been sieved so that it remains nice and open.
That's the basic, simplest advice. From there we move onto those with more specialised needs, for example that need more moisture retention. eg Paphs and phrags. There you can start to add moss, but it pays to use it as a layer on the bottom covered by sieved fine bark. If the moss is exposed to the light, sooner or later it will go green and slimey.
From there you can expand it to cover all needs as you learn more.
One point, orchid pots need to be shallow. Diameter is irrelevant, it is the depth that matters most, as with a dense, water retaining mix like fine bark and moss, ESPECIALLY if they are mixed together, depths of more than 4" increase the risk of anoxia and root rot.
Basically you can grow orchids in dog turds if you know what you are doing, but I wouldn't recommend it to a beginner.
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