I installed the small 18 inch Furuno dome last year and have been wishing that I'ld gone with the larger unit because of better resolution in bad weather conditions. I was then happy to read Maine Sails take that under 2 nm it makes little difference. The reason that I went with the smaller dome in the first place besides a difference of around $500 in price was the smaller profile under extreme wind conditions. The dome is pole mounted on the stern so tacking the sail is not the issue, but the one hurricane that I've been in has me thinking about wind resistance.
So that was my reasoning or rationalzation. The weight issue was not a factor nor the power useage since there is not much difference between the two units. I just hope that I haven't given away much performance under the 2 nm range.