Migration from one Macbook to another
The Migration Assistant was made by Apple for one purpose only, so it is the best way to migrate from one device to another.

As for the 10.10 since it is still in beta phases, the Migration Assistant on the host computer might not work as it should, since beta phase testing is not completed yet.

Since 10.10 has it own way of managing the files ect... the transfer to 10.9 might have problems with that.

10.10 Release notes: 

OS X 10.10 Installation
Known Issues
Migration Assistant may hang while transferring your information from a Time Machine backup. To proceed with the login, quit the Installer.