Sound quality on initial test:
XLR > S/PDIF
Both are very good, but just that little bit better for XLR. Could be a tweak here and there to get them equal but I think I'll probably stick to XLR.
I would expect you to find this for the following (somewhat complex) reasons. I only use SPDIF for reamping where I send it from the Kemper to my DAW and then back to the Kemper for conversion back to analog sound. For regular recording I use the XLR outs to my audio interface. The reason for this is outlined below.
What you have to think about in all of this is signal chain and conversion between digital data and the analog world we all call the sound we hear.
In the case of using the Kemper to the DAW with SPDIF you are transferring the Kemper sound directly as Digital data which your DAW then records directly as digital data - later your audio interface (or Kemper in the case of reamping) does the conversion (interpretation) of the digital data into the analog sound you ultimately hear.
When you take the audio from the Kemper's XLR or TRS type outputs you are taking analog sound from the Kemper and then using your audio interface to turn it into digital data that you then record and then later turn back into analog (sound) when played back again through your audio interface.
The thing to keep in mind is that the conversion from Digital to Analog (D/A) is not an exact science - there is some (a lot really) audio art to it as well. The reason for this is that digital is not analog and analog is well analog and the interpretation of what is missing between the smooth analog curve and the stepped digital data is subject to interpolation - no matter how many bits or what sample rate - more bits and higher rate always gets closer but is ALWAYS still an interpretation. There are many factors involved in the D/A process that can ultimately change the resultant analog wave forms (interpretation) of the digital sound - again this not an exact one way to get there type of thing - hence the art of the audio conversion process.
So in the case of the Kemper guitar stack sound we all know and love I personally believe the Kemper has a better D/A process and conversion of its digital sound back to analog than the most (any) audio interface I have heard does.
Because of all of this I believe taking the Kemper's analog output (XLR or TRS) and then converting back to digital through your audio interface (assuming it's a very good one) will produces a better warmer more authentic sound than taking the direct digital data from the Kemper (SPDIF) and having your audio interface do the conversion back to analog later.
In the case of D/A's found in most modern high end audio interfaces this difference is not huge but I really think you can hear the difference between them and the D/A's found in our beloved $2,000 Kemper box. If you record SPDIF and then send it back to the Kemper for final conversion to analog (as is the case with reamping) you are using the Kemper for the final D/A step and thus is the best use of SPDIF in my opinion (as a studio guitarist and engineer). Otherwise I use the beautifully lush and very analog sounding output the Kemper has on its own and then capture that with your audio interface A/D's for recording into your DAW.