I2s is the best way to go because it offers greater resolution and can handle DSD which SPDIF usually cannot.
I would only use I2s if it is a native output to input, otherwise USB or AES would probably be better.
IME RJ45 is preferred because of the larger conductor size but I found no benefit in going below 0.5m in length, whether HDMI or RJ45. PS Audio recommends 0.5m.
I had excellent results with WW Platinum, Furutech was good too but darker.