Just to clarify (since I couldn't edit my post above for some reason)..

The balanced input jack is 1/4" TRS and the SPDIF input is digital coax 75 ohm.

It's not a very meaningful question.

Assuming you are talking about a proper audio interface and not a computer sound card, the S/PDIF input will have no sound of its own, as the digits pass unchanged through it. Any sonic qualities will be attributable to whatever piece of equipment amplified and digitised the audio signal ahead of the S/PDIF input.

By contrast, an analogue input (TRS jack in the example you give) will feed an input amplifier of some sort followed by an A-D converter. Both of these components inside the interface may have a sonic quality of their own, and this will be added to (convolved with) whatever was present in the incoming analogue signal.

If you have a 2-channel high-quality digitising pre-amp, you can feed its S/PDIF output into a computer using a cheap interface that happens to have an S/PDIF input. As long as the interface does not corrupt the digits on their way through, it will cause no signal degradation.

I would exclude most PC motherboard audio and S/PDIF inputs from this description, as they can cause all sorts of problems.

Boswell, post: 441634, member: 29034 wrote:
Any sonic qualities will be attributable to whatever piece of equipment amplified and digitised the audio signal ahead of the S/PDIF input.

Interesting. What "piece of equipment" would you say would be good to have "ahead of the SPDIF input" ? Can you give me an example of something you would use personally ?

Boswell, post: 441634, member: 29034 wrote:
If you have a 2-channel high-quality digitising pre-amp, you can feed its S/PDIF output into a computer using a cheap interface that happens to have an S/PDIF input.

I'm assuming this is what you're referring to by "audio signal ahead of the SPDIF input". High-quality digitizing preamp ? Like.. for example ?

Boswell, post: 441634, member: 29034 wrote:
Both of these components inside the interface may have a sonic quality of their own, and this will be added to (convolved with) whatever was present in the incoming analogue signal.

Again, can you give me a good example of something that would have a "sonic quality of it's own" ? In this instance, it would be TRS related though. Something like a PCI card perhaps ? What would you say has good "sonic quality" ?

jp008, post: 441654, member: 50008 wrote: Interesting. What "piece of equipment" would you say would be good to have "ahead of the SPDIF input" ? Can you give me an example of something you would use personally ?

Another question that's impossible to answer without any idea of what you want to record and your budget. Since S/PDIF is a two-channel digital transmission protocol, you would be looking at two-channel pre-amps that have S/PDIF output, an example of which would be the Audient Mico. That particular unit has been discontinued now, but there is still stock around at dealers.

jp008, post: 441654, member: 50008 wrote: I'm assuming this is what you're referring to by "audio signal ahead of the SPDIF input". High-quality digitizing preamp ? Like.. for example ?

See above.

jp008, post: 441654, member: 50008 wrote: Again, can you give me a good example of something that would have a "sonic quality of it's own" ? In this instance, it would be TRS related though. Something like a PCI card perhaps ? What would you say has good "sonic quality" ?

This would be a pre-amp that imparts a recognisable quality to the signal passing through it, as opposed to being "transparent". Transformer-input pre-amps are best known for their desirable sonics. Look at the API 3124+ as an example.

Your comment about a PCI card is disturbing.

jp008 : You need to give us the whole picture. If I ask which kind of fuel is better for my car and I don't say which car, I'll get various answers.

What is your audio interface, (make and model) ?
what do you want to record, vocals, keyboard, drums.. ?
what other gear would you like to use, outboard preamps or compressors ?

In my situation, I have Focusrite ISA preamps plugged in my TRS line input of my RME FF800 and I use 2 Universal audio LA-610 which go to a mitek AD96 converter and then to the RME in SPDIF.

Be aware that using any digital port on your audio interface implies some synchronisation process. either with wordclock cable or via the signal itself. So one of them needs to be the master clock and the others are slaves...

What's better, a car or a boat?