Gardner - this is a great question because it can cause us to focus on a design that might be close and compare. We are from Toronto Canada and I went out to see Rob 8 years ago when it was just Harry Proa and the whole family had not been introduced. I went on the boat and it was very cool but a bit to get used to. The design has evolved and every year more and more of the strengths are apparent - in other word it grows on you.
So here is my 2 pennies worth. I see at least twelve areas that this design has going for it. So if you were to take a cat with 4 double berth and compare that to a Rob proa with similar number and size of berths these points would emerge. Some are overlap but worth mentioning. Rob might say that my percentages are a little low but they are my guesses.
1/ Cost 20% less to build or have
2/ Weight 20% less at least (thus saving money)
3/ They say there is a magic to sail boats when they hit the 60 plus ft mark in terms of handling at sea. The comparing cat would probably be under 50 ft.
4/ Bridgedeck clearance is not as cortical on the windward side because the hull lifts
5/ Build time 20% less at least
6/ Windage is less because the boats can be lower
7/ Manoverabilty much better
8/ Usable space - looks bigger - Cats have front and back cut off by the main salon - with the newer charter design I would think have a "big wide open look' - down below there is more freedom to lay out the space
9/ Better shelter and visibility underway - some could argue this point.
10/ Safety - many have talked
about this point so I won't repeat
11/ Speed - one driving hull - drag etc.
12/ Ease of build - Rob is fixated with this
Last and not included is the cool factor of the boat sitting on the water. like I said it grows on you.