I have both the modded MK1 and a MK2 with Nickel transformers.
I can honestly say that I feel the MK1 sounds better than the stock MK2. With the nickel transformers installed in the MK2, I feel it is just slightly better sounding however, I prefer the touch screen over the new interface in the MK2 and also not dealing with the issues the MK2 has currently. The firmware updates have yet to correct these issues as well as supporting all the features promised. Many features still do not work or work correctly.
The MK2 USB input is better sounding than the MK1 however I just use the i2s input and install a shorting plug in the USB port. Also, the USB input on the MK2 has some issues with some streamers that cause a loud noise when switching from DSD to PCM.