Hi Michael
the answers to your questions will not, in my opinion, give you a good indication of which will be the best boat for your individual requirements. I will give my opinion on the questions you asked first. Bear in mind I have got the 480 and while I will give you accurate information about that boat only I am not saying it is the best for you
OK, your questions: Stable: the 480 with its low centre of gravity because of the biplane rig. Also if it is rigged like mine the sails can be furled or reefed in a few seconds without letting go of the helm.
Faster: in theory of course the boat with the greatest waterline length should be the fastest. But if the boat is loaded up with cruising equipment and is fitted out with a toilet and galley (kitchen) and there are 3 or 4 people on board, then the biggest boat may not be the fastest. For example if you are doing day sailing and carry only the supplies needed for the day, and if there are only 2 people on board then that boat, whichever model, will possibly be faster. Multihulls get a lot slower when they are heavily loaded as Bernd will tell you.
But look, these questions are only part of what you need to decide. You need to decide if you are going to do extended sailing including summer holidays, not just overnight. If you have got access to a nice place to put the boat on a mooring like David has with his Eco 6, then you can avoid owning a trailer and the associated costs. If you are going to keep the boat on a trailer and launch it every time you sail you would be at an advantage if you dont have to travel too far to launch the boat.
If you are going to launch the boat every time you sail as I do then I guarantee the 480 is far, far quicker to rig. The 520 of course is a new design so we havent seen it yet. But I have been lucky enough to visit David a couple of times when he was building his Eco 6, and also Hamish with his Eco 5.5. They are both beautiful boats and the builders did a great job with them. So I am not reccomending the 480 over these other boats, i really like them all. By the way we also had a KD650 built locally here as well which I sailed on many times and that is a great boat too if you want something a bit bigger.
I hope David may contribute to this discussion and tell you more about the 6. He is also a vey experienced sailor and has published sailing and cruising books so he knows what he is talking about. Any boat with a cabin takes time to build so it also depends on your personal experience. For example David built his 6 quicker than I built the 480.?
regards
Bryan
bryanandmarycox