Originally Posted by Rambo
Its essentially a synonym but I'd define pace as speed + depth + movement. Shots that keep the player moving constantly and the ability to move the ball around the court while still having power behind the shot. Not just hitting the shit out of the ball.
The main difference is in the description of the players as a whole package... If you are describing a player defined by their power (because their game lacks other tact) then often they are described as a 'power' player. Carlos Moya and his FH come to mind. But as semantics go, announcers will very often say of say, Roddick, that he has a lot of 'pace' on his FH.
Pace, is in IMO used to described more well rounded players that merely have pace as a result of proper technique and developed skill. Almost a side-effect to proper play. That said, pace never necessarily means 'fast'.