How large is the room? Do you listen to music at higher volume levels? How far is your mail listening position from where these speakers will be placed (remember, they'll be near a wall so account accordingly)?
Either system is great. Both have the ability to EQ. So, I wouldn't worry about tonal characteristics too much. The one feature I like about the Kii is that it has BT built-in. That's a nice feature to have (despite audiophiles hating it, I rather enjoy the convenience at times). ROON will soon be arriving the DD8c, finally.
Utlimately, IMHO it boils down to this:
Kii THREE is more compact and has a neat little remote with a lot of I/Os and BT as I mentioned.
DD8c can play louder with more bass output (no built-in limiter whereas the Kii has such) and soon shall have ROON.
How much does aesthetic factor in to your decision? If the room is small and you don't necessarily listen loud then that really may be the final determining factor. It's a tough call, for sure. Hard to go wrong either way as long as you understand the limits.