I have a cutter
rig and the dome is installed on the mast just below the spreaders ---with the additional height you get much better range and 'picture view' and it doesn't cause any problem when tacking. If your boat is a small or light boat (little displacement) or has a very narrow beam then you might consider whether the weight of your radar mount might adversely affect the stability of your boat---probably not a problem but maybe think about it.
As for where to mount the display for me there is no question that it should be mounted where you can reach it, where you can see it and most importantly WHERE YOU CAN ADJUST IT without having to leave the helm.
Mounting it down below is a waste ---imagine coming into a port or anchorage in the fog
--you will want to be able to 'see' with your radar and to quickly react --you can't do that if the display is down below.
Also radar is a dynamic instrument --not a 'turn it on, set it and leave it alone' instrument --you will need to be able to adjust or re-tune it as conditions change --for example you will need to adjust it because of more or less 'sea clutter', or rain, or heavy fog, or the need to adjust the range to identify objects either nearer or further out ---and you don't want to have leave the helm (even with auto-pilot) because the reason you need to adjust the radar unit is because conditions have changed or sudden increased perils (ie. some stupid SOB in a high powered boat screaming along in the fog towards you)
THAT MEANS THAT THE DISPLAY MUST BE CLOSE AT HAND AT OR VERY NEAR THE HELM.
We sailed down from Vancouver to Mexico
in the fall of 2005 with fog all along the way until after San Diego
. With my radar display and control mounted above the binnacle at the helm we could (and regularly did) adjust our radar numerous times and by being able to do so we were able to save our bacon numerous times.
Just remember you will most need your radar when things are NOT all sweetness and light but instead are gnarly, difficult, dangerous and stressful.
For me the ONLY place for the radar display and control is at the helm.
Just my 2 bits FWIW