Football Outsiders has been using the "speed score" as a metric since 1999; here's an explanation relative to the 2008 NFL draft and here are the 2009 results. What follows regards the 2010 NFL scouting combine.
The difference between a 40-yard dash time of 4.37 seconds and 4.43 seconds is 60 milliseconds -- just about half the time it takes the human eye to blink. To most people, it's imperceptible.
Because of a historically effective metric suggesting that Tate might end up the better pro back.