With the 520 you can have the option of the bi plane rig if you prefer. After building and owning the Duo 480 the bi plane rig would be my first choice.
cheers
Bryan
ps the Eco 6 is a great boat. I was lucky enough to have a sail on David's boat and I cant praise it enough. Rod is going to love his boat when he gets sailing. Of course it is a bigger and heavier boat than the 520 which may be a factor if regular trailer travel is anticipated
bryanandmarycox