It is obvious that Stephen owns a H-18. I have owned both and sailed both and my only choice would be the nacra 5.8. My reasons are
Boomless rig
Solid mast
no roller furler
travler system
mainsheet system
rudder lockdown system
more positive bouyancy forward
Harken standard equipment

In my opinion the 5.8 out performs the H-18 hands down. They are both very fine boats, and I am sure either one will give you and your family years of pleasure.