Why not the mast? After lots of research, that's where we mounted ours on our Bristol 38.8. What it came down to after doing the research was that the added height on the mast gave better performance. The argument about scanning the sky for airplanes and the sea for fish when heeled over made a lot of sense. But I don't see a loss in performance when we are heeled over. I also don't see a loss of performance in detecting close range items. I can honestly say that it has never missed picking anything up. We have the Garmin HD scanner.
When I was thinking about it and looking into the best options, I wasn't concerned about the added costs of installing on a pole or on buying a backstay mount. I was willing to pay for the best solution and weighting and comparing trade offs. I understand physics and know what a moment is, so I understand what added weight aloft can do too. I ended up installing on the mast because I felt that gave the best performance and I don't regret it.