Personally I don't see much benefit to a 48 mile dome over a 24nm or 36nm, in the case of the 18" Garmin, for on water distance. I prefer an 18" dome size for obvious reasons on a sail boat and currently Garmin packs the most performance into an 18" dome. The GMR 18 is a 4kw 36 mile unit but it's 18" isnead of 24".
As you go bigger in dome the screen resolution does get better because the beam width is narrower. I have used very large, and very expensive, 72 mile radars on mega yachts and large fishing vessels and the screen resolution is great but you still can't see beyond the curvature of the earth. Anyone who claims they can see anywhere near 24nm, 36nm, 48nm or 72nm, on water, is full of bull excrement.
What you can see with a bigger dome is weather but with the quality GPS weather subscriptions these days why bother adding all that weight & expense?
Beyond that the new High Def 18" domes are amazing! Furuno & Garmin have them & I believe Raymarine will. My Garmin was very, very sharp right out of the box and after a little tweaking matched resolutions and target displays of units much larger, and more expensive, that I have used.
I use radar a lot, being from Maine, and probably have well over 2000+ hours, perhaps more, (was a commercial fisherman for about 8 years in my younger days) and literally thousands of miles of time spent in the fog with radar. My suggestion is quite the opposite of dogs. Get the dome on the mast and get it high.
The junk Dog and I banter about with close in targets is just that junk, especially when it comes to the real world!
I have picked up kayaks at close range and many other close in targets with a dome on the mast. In my life there have only been a few instances, perhaps one or two, out of thousands of hours in thick fog, where I had so little vis that a lower dome would have even mattered. In contrast I have had hundreds of times when more range would have been better.
A dome, regardless of it's rated range, 10' off the water will see a 10' tall target at about 7.6 miles.
A dome 20' off the water will see a 10' tall target at roughly 9.2 miles.
Most mast mounts are roughly 25 feet high or more!!
Never under estimate the distance you'll need. We have a ferry up here called the "Cat" that runs at over 50 knots. A 6nm dome visibility is NOT enough in pea soup for a vessel traveling 50+ knots trust me.... If you sail in shipping lanes or high traffic areas you'll want as much distance as you can get.
The higher the better!!!
The first photo bellow shows our old boat with mast mounted radar. The second photo shows my neighbors boat which I could pick up clear as day with that mast mount. If you have not seen a target on your radar before it gets that close you have MUCH bigger issues like going back to the owners manual big!
Even with my spar mount on my Catalina 310 I could make out this neighbor, the C&C just behind my boat, clear as day and he has no reflector...
Our current boat has a pole mount radar and I will eventually move it back to the spar for better range performance. Keep in mind that a pole mount ten feet off the water, in ten foot seas, will lose targets and range performance every time you are not on the crest of a swell of a wave and when you get to the bottom of a 10-12 footer you will have zero target tracking ability until you begin to come back up and reach the crest again where the best performance will be. The has happened to me more than once with low domes.