I will cast my vote in favor of the Beneteau,
especially if it is the deep keel, tiller steered version.
I have sailed on, and raced against several Catalina 30s
although not a Mk II version. They sail fine in good breeze
and flat water, but are slow in light air and chop and must
carry a large headsail to stay powered up. The C30 is really
a retread of a early '70s design, so I would be expect the
B310 to perform much better in most conditions.
Islander 30 II 'COOL'