Referal keys you can sent from your account on the sto homepage.
Buddy keys come with your gamecopy.
The difference is a referal allows a player to play "indefinitely" but only flying around in Sirius sector block and playing the Azura mission.
A buddy key allows complete game access but limited to 10 days.
I'd say the buddy key is more interesting, as it allows your friend to explore more aspects of the game. If he isn't hooked after ten days it's not for him anyway
I still got a buddy key left which I really like someone to use so i might eventually get the holographic boff.