It depends...
In theory Gigabit is faster, and it's certainly more flexible as you can access the same drive from multiple computers. You can also have longer cables between the computer and drive.
In practice, most cheap gigabit network storage boxes can't reach very high speeds and USB would therefore be quicker.
USB also has other advantages in that you can easily boot up off a USB drive connected to an Intel Mac.
So I would only go for gigabit / network drives if:
- You have multiple computers that need to access the drive at the same time
- You want to hide the drive away somewhere
- You don't care too much about access speed
- You want to spend a lot on a computer based server, where you WILL potentially get better speed from Gigabit