That is really not an easy question to answer. The specs on paper of the Amp Ultra seem to be better (100W versus 80W) but the audiosciencereview measurements of the c268 show that the burst power (powerful musical transients) is above 200W for the NAD. So if you are listening now at moderate volumes at most (so not near full volume), you are not power-constrained and the peak power might not be relevant.
Your speakers will be the weakest link in the chain concerning distortion and noise. So probably you won't be able to hear any differences between the NAD or WiiM so pick what you like.
For user convenience, the WiiM Amp Ultra wins of course because the delay is not there. But you can also save the ~ 600 euros/dollars because you already have a great sounding setup ....