As this thread has demonstrated, it seems impossible to say for sure which is the more impressive, there are far too many different factors at hand.
Watch Dogs has set a new industry sales record for launch of a new IP. That, to me, is impressive. MK8 has sold 1.2 million in its first weekend on a single console that has a user base of 6 million. To me, that is equally impressive.
Both have shifted consoles; it could be argued that MK8 shifting WiiU consoles was a more difficult task, but then MK8 has the weight of a huge franchise behind it.
4 million over several consoles in a week versus 1.2 million on one console on launches staggered over 4 days can't be compared - it's apples and oranges. Likewise, the current attach rates can't be compared yet, because of Watch Dogs's head-start.
I think a few months are needed before I could say for sure which impressed me more.
Three and a half weeks in, and after some adjustments - WD was adjusted down by 600k and MK8 adjusted down by just under 100k on the May 31 figures. So that's 3.4 million for WD on 4 consoles + PC and 1.1 million for MK8 on one console.
In the first week, despite only being out for a few days, MK8 outsold individually WD (360), WD (PS3) and WD (X1). From Week 2 onwards, MK8 has outsold WD on every individual platform. Looking at the latest figures, (June 21) WD has increased by 1.2 million to 4.6 million across the 4 consoles, and MK8 has increased by 0.7 million to 1.8 million on one console (subject to future adjustments, of course) closing the gap by 0.5 million. (WD for PC not included in June 21 calculation as it has dropped out of the top 75)
At this stage, (according to my maths) attach rates for latest gen are MK8 (Wii U)28%, WD (PS4) 25%, WD (X1) 21%.
It will be interesting to see how things continue to pan out over the coming months.