Coax or USB between transport and DAC?

It’s been a long time since I had digital separates and I am thinking of getting a DAC to use with my Pioneer transport. Which cable offers better transfer, a coax or USB or is the difference minimal?

I would use coax; USB connections appear to be more variable than coax.
If you're using USB, your source is a computer, and quality here depends in a big way on the USB implementation. Poor USB circuits at the DAC, when fed from a laptop, will get beaten by coax coming out of a decent CD transport. In my experience, most USB DACs under $1000 are limited to 16/48 over USB, and have poor implementation. To get really good sound, you need a decent USB/SPDIF converter, in most cases, unless you're spending over $1000 on the DAC. Suggestions on a DAC-only USB solution? Benchmark DAC-1 USB and Wyred for Sound DAC1 or 2 is where I would start.
I bought a PS Audio DLIII, and the laptop-DAC path via USB could not compete with my Denon changer as a transport. I bought a Vlink and the Jplay plug-in, now the sound is a little forward, but I have a wider soundstage, better depth, more detail. Planning on upgrading the converter.
I doubt your Pioneer transport can do USB? USB is only for computer sources and works very well or very poorly depending on the design of the DAC. Coax or toslink is what you'd be considering. Either can work very well.
If you were to buy a Wyred 4 Sound DAC-2, it has an asynchronous USB connection and built-in volume control (as well as many in's and out's)... connected USB direct from any PC or laptop will kill any COAX connection you try. Forget the transport idea completely. Rip all your CDs as 24/96 or 24/192 WAV or FLAC, it will sound far better than using any CD player. I had a DAC-2 but recently upgraded to a LINN Akurate DS, no more discs!!!
Audiofreak32, CDs are 16 bit and ripping them at 24 bit will not give you any benefit. Then, IMHO, FLAC is the way to go if you want to manage the library in a convenient way because WAV doesn't support the tags.

Ripping at 24 bit makes a significant difference. Have you heard a 24/96 or 24/192 WAV or FLAC file off a LINN DS? There are not many CD players made that will compete. The key is eliminating the transport all together. They are comparing a LINN Akurate DS to CD players in excess of $20k.
Audiofreak32, I'm a Linn dealer and one of the first to embrace wholeheartily the DS bandwagon (at least here where I live and work)....... so I know a little about it. A different matter are "real" 24 bit files but one CD (that is 16 bit) ripped at 24 bit will not become a real 24 bit (I mean real high reolution file). I've tried to rip some CDs as 24/96 files but IMHO is a waste of time as anyway the Linn DS is doing this same job internally. Here is an excerpts from a thread on the Linn forum posted by a Linn DS engineer: Internally, the DS uses 35-bit precision for upsampling and digital volume control. There is then a single, properly dithered truncation to 24-bits just before the DAC. External upsampling introduces an additional truncation (to 24 bits) so the overall processing chain becomes more lossy.

Happy listening with your DS and yes, no more CD player for me too.
If you are saying that by ripping at 24/96k or 24/192k FLAC or WAV, then sending those files into the DS, will result in a "lesser" sound quality or "more lossy", I will have to disagree. I was told that ANYTHING you send into any DS model, it will decide what to to best and do it (the non-technical" response).

Both LINN dealers in Arizona (one being Steve Carroll from LINN, the other Kurt Kenobbie who has been selling LINN for 25 years) would say to rip at least 24/96k FLAC or higher.
"If you are saying that by ripping at 24/96k or 24/192k....": no, I've not said so, a Linn DS engineer said so. Again, there is no advantages in ripping a 16bit CD to 24 bit when your DS will do this job on the fly during the playback. Please read carefully the explanation from the Linn guy.

I have found that the 24/192 files do sound better than lesser bitrates when played on my DS. Again, a real world listening test vs. a technical explanation from an engineer. I have an engineer friend that also told me that it is not possible for an LP to sound better than a CD.