As alan wheeler says "to do what"
As this is a cruisers forum I assume you mean cruise
In my opinion, waterline length is the most important aspect for comfort at sea, the longer the better.
In an under 40 ft catamaran
the usable accomodation available is poor at best plus the bridgedeck is either too close to the water
or there is no headroom
on the bridgedeck. Plus the hulls are just long thin, narrow tubes? A 30 ft catarmaran is a daysailer.
With a Trimaran
configuration you get more usuable room than a catamaran in the 35-40ft range plus they are a lot cheaper too build and fit out.