As someone who didn't think W_D was great, but enjoyed the mechanics enough to finish a lot of the game:
- Mafia 3's story and driving are much better (simulation driving anyway, I haven't tested the more arcade setting).
- Watch_Dogs' combat is a little more fluid because Ubisoft's animations can be top notch, but Mafia 3 is way more swift and brutal. You feel like a powerhouse, and weapons have good weight and recoil.
- Mission structure is equally uninspired featuring the tried and true district takeover setup/collectathon we all know and love, but it ties back into managing your underboss relationships/rewards as a twist on the concept. Main missions are more intense and atmospheric.
Mafia feels more focused and immediate, and the narrative is on a different level completely. Certainly has its fair share of bugs that need fixing, but I'd still say it's a better game than W_D at this point.