Is fan hyn used more in South Wales? I have always heard people say yma here in North Wales.
I use both (I'm northern) and really they mean slightly different things to me. "Fan hyn" means "This place" if translated directly, so I would say "Sefwch yn fan hyn" for "Stand here" since I'm being specific to a spot, whereas I would say "Arhoswch yma" for "Wait here" since I would be talking more about a general area. Not sure if these are the actuall rules just how I personally use them.