I have never used the Farrell rulers, but have had good luck and good service with the Westalee with both my domestic and mid arm machines.
The Westalee Starter Set, where you get the foot and the curve/straight edge ruler is my still my favourite ruler of all. They have great educational videos and good customer service and when I went to a midarm and sold a bunch of my domestic rulers, (and put the money toward more Westalee rulers for my mid arm) they held their value.
Watson