I have sailed on both boats and raced on my C&C 29-II against the Beneteau. Observations, B 28.5 has more room in interior. One is ailed on had a tiller with slatted seats in cockpit. Usable room in cockpit was lacking. The 29-II interior is more compact but more room in cockpit because of wheel. The 29-II has much better sailing capability with a slicker hull design, better foils and larger sail plan.
If encounter chop and lighter air, the masthead C&C goes to weather significantly faster than the 28.5.
You can find good condition 84-85 C&C 29-II in the States for around 17K -22K. With the Loon currently worth more than the dollar there are good deals to be had.