As Phil says, entirely personal preference. DSD or DSDx2 Upsampling will give a different presentation to using DXD but one is not technically better than the other. DSD or DSDx2 tends to be smoother and perhaps a bit more musical to some folks, whereas DXD is a bit sharper on transients and more dynamic.
Provided that your Vivaldi Upsampler has DXD Lock turned on, you’ll be able to select DXD on the front panel of the Upsampler with the Output button. Not sure why it isn’t showing up in Mosaic, I’ll get that followed up.
The difference between that and selecting, for example, 384000, is that with DXD the rate changes between 352800 and 384000 based on the sample rate of the content you are playing. It keeps everything running in direct multiples, which is good for sound quality.
After the conversation I had with Phil, I selected 352800 in Mosaic. But I noticed that sometimes It was 384000.
I think the Upsampler works as you mentioned but it is not displayed in Mosaic the way it was with the previous release. I might be wrong, but I guess you have the possibility to check that with the Mosaic developer.