The Thunderbolt cable has become the new very high speed standard for the last generation of Apple computers (Since roughly the arrival of Intel i5 / i7 processors). This cable allows between 10 and 20Gb / sec according to the version supported by your mac and of course perfectly compatible in both cases. For cons, the flow of the Thunderbolt will be used only in specific cases (Your hard disk will be the limiting factor for transfers) so the firewire option may be cheaper if you still have this connection.