The RJ45 connection is another way to pass i2s. I don't have experience using the RJ45 connection, but I have used the HDMI connection between the Gaia and Terminator Plus.
I have an early Gaia and early Terminator Plus, and with the firmware update, the i2s connection between these two boxes is no longer compatible, so I have had to switch back to AES. Before the firmware update, I preferred i2s over AES, but I do like the firmware update so I'm living with AES.
I also like DH-Labs for reasonably priced digital cables. I'm currently using their AES cable.