The inner scale on the Douglas protractor is one of the simplest and most brilliant inventions ever and allows you to measure bearing lines on the chart directly without drawing the line. It also allows you to place the protractor on the chard and use the edge to plot a bearing line from a point or to draw in a track and measure off its true track directly all in one go.
You can also use the square lines on it to act as a parallel rule.
Those crappy ones with the 180 out scale are obviously designed and produced by someone who does not know what the real Douglas protractor is for.
I did use the Brady Rule but found it a little "busy" and also the construction was somewhat flimsy. I remember seeing many of them broken and taped up with sticky tape. Also, people used to cover them with clear Fablon to strengthen them. Perhaps the new ones are more robust - I hope so.
Happy landings!