You don't see a lot of Machins in general bullion circulation in my experience. I recently got a handful in and they flew out again so fast I made a note to put up prices a bit higher next time!
When you compare the official mintages (whether these are to be trusted or not I know not) there are around 37 million for the Machin portrait compared to 315 million of George V, and while more of the GVs will by now have been melted down, I'm sure they still vastly outnumber the Machins.
Until recently I assumed the portrait by Arnold Machin was the same as his portrait on the stamp (the most reproduced artwork in history), but in fact they are distinctly different, not least as the portraits face different directions!
I prefer the Machin portrait to the Gillick portrait, which I find a bit "soft", but not sure if that's down to the engraving or the strike.