If Watson has confidence at the moment ringing-in is allowed, it seems it will always win. So how much time it has to achieve that confidence, as a function of how the text is fed, may be more important than buzzer mechanics.
Based on the same general idea of a common starting point that motivates waiting until after the entire question has been read before allowing any ringing in, I could see there being a tiny 'common period' where all buzzes are considered as coming in simultaneously, with the person chosen to answer then being chosen in round-robin fashion (or for maximal drama, favoring whoever is behind). After the tiny common period, it would be strictly based on first-to-buzz.
It would take away a twitch-timing factor that has been important for human champions, too, but offer more fairness with regard to computers and even people with slight ticks or timing problems.