Which sounds better: I2S or Coax.

Correct.  There is no industry standard, so it is proprietary to the manufacturer.  In the case of PSA, its SACD transport is designed to work with its DAC using I2S (HDMI).  That is just how it is.  

When I first purchased the PSA SACD transport I was not using a PSA DAC.  I thought any DAC with an I2S connection should work.  Not to be.  I the purchased a used, 10 years old PSA DAC.  Not to be Part 2, as the new SACD transport is not backwards compatible with the 1st (?) generation DACS.  

Great support from PSA every step of the way.  Paul spent many weekend hours with me diagnosing what was going on.  At the end of the day, PSA offered me a tremendous deal on a new DAC.  So, all is well and I am extremely satisfied with PSA and my set-up.

Keep in mind though, this is how PSA has designed its SACD handshake.  As a side note, I do believe there is an 'Ask Paul' YouTube video out there on I2S.

Rich  

The i2s is way way better in my dac transport pair. Not even close to any of the other connections.  

I used a Sony UBP-X1000ES with the Chinese HDMI Video to I2S HDMI adapters from China with excellent results.

I noticed far more difference between Toslink and coax through a Topping E50 than any inputs on a DirectStream.

Of course you NEED to use i2s with a PS Audio transport for features. My rec is Revelation Audio Labs HDMI style i2s cable.Good luck getting one of those. Start praying.

If you want to use the second i2s input, the Matrix SPDIF-2 is good enough for me.