Of the places you mention, I'd pick Xanadu -- nice condo units (though a little strange looking), nice new pool, good location.
Casa Caribe is no more. It became Avalon, which had big timeshare plans, but they've apparently fallen through and who knows what will happen up there.
Mosquitoes are rarely a problem anywhere as long as you're on or near the water and a breeze is blowing. Back away from the water on North Ambergris and also on the far south end of the island they can be pretty bad.
--Lan Sluder
www.belizefirst.com
AmbergrisCaye.com 
