I see the decision is mostly unanimous, but let me go against the grain and vote for MK8.
I say this because MK8 is an established franchise while Watch Dogs is a new IP. Sure, the hardcore crowd will buy Watch Dogs in droves, but we are the minority. The majority of gamers are casual, and Watch Dogs isn't a game they will know much about. However, Mario Kart is a game they know.
Watch Dogs being available for 5 systems simply spreads it's sales thin (thin, as in over a larger field as opposed to on a single system). There will be people who own both a PS4 and Xbox One but decide to get the PC version; and of course vice versa. However, MK8 is only on one console - and even though it's only on one console, it doesn't necessarily mean it can't outsell a game on 5 systems. There have been many examples of this with many other games.
