The R3A is more popular only because today more people are used to using AE cameras,
not mechanical.
As my page on the R2M / R3M explains
http://cameraquest.com/voigt_250.htm
I personally prefer the mechanical cameras.
"All of that said, on a personal note I have to say that I prefer classic mechanical cameras over electronic cameras. So my pick would be the R2M or R3M over the R2A or R3A. I just like the sound of mechanical shutter camera better, as well as preferring the classic metering readout."
Stephen