Is it better to mount the dome on top of the mast or low off the stern? Will the movement of the mast offset the height and convenience of the mast mount? Is the stability of the lower stern mount above bimini a better option?
It depends on many factors.
Just remember that the roll of the vessel will be the same up the mast as at the stern so if you had Domes mounted in bother locations, they would both sway in the same way.
If your stern mast is high enough, the location would make for a nice any easy install.
Installing on the lowest spreaders would give you better range but it can sometimes be very hard to run new ethernet cables to this location.