"...for serious blue water sailing..."
But where are you actually going?
Space, weight and budget
are all major considerations for small-boat/blue-water sailors; so the first question shouldn't be 'which' radar, but 'whether a radar.'
We spent nine years sailing a 27-footer around the UK, across Biscay, around the Iberian Peninsula, then the length and breadth of the Mediterranean
. The boat had no radar and whilst there were days where we stayed in harbour/at anchor
because of poor visibility, I can think of only three occasions in nine years that we thought: "Damn, wish we had a radar."
boat, which we've sailed from the eastern Med to S. America, Caribbean
, and the east coast
USA as far as Maine
is fitted with a radar as there was one on the boat when we bought it. That radar (an old Furuno) has been used in anger once in three years; not in Maine
either, it was off Fernandina Beach about a week ago. Again, there have been days where we've not gone out due to poor visibility, but even with the radar fitted, we wouldn't contemplate going to sea if we needed it so 'see'.
Everyone has their own needs/priorities and if we were a 'working' boat and obliged to go out come what may, then yes I'd want a radar, but on a 'cruising' boat, radar isn't high on my priority list.