I have the the 424HD. Raymarine tech at the FLL show explained that the "D" was basically their previous analog radar with a digital interface, while the HD had the full digital signal processing, etc. I've no complaints about the HD, and it does image storms on land and water
. The colour images
of storms look remarkably like what you see on the Weather
Channel on TV.
I also used the 10m cable to run down the mast, with an extension from the mast to nav station. It's really the only way to install if you ever plan to take down the stick. You don't want to even think about splicing that cable.