Keychain Access the 'login' keychain is the user names and passwords that you have stored for the current logged in user. The
iCloud keychain is as the title states stored in the cloud available to be accessed on any devices under the same
The main risk with storing anything within a cloud service is the risk of these online services being hacked.
OS/X devices are multi user environments, and under these setups often share an
iCloud account, or are using
iCloud Family Sharing feature. These features could lead to passwords being available to users/family that you would not want to have access to.
As for copying the items over, as far as I am aware there is no way of doing this, looking within
Keychain Access application there is no option for this. Also within preferences there is no option to set the default location, any new passwords will be stored within the 'login' keychain.
Hopefully this has cleared things up for you.