sampras is probably the stronger of the two, maybe the slightest of edges in straight-line acceleration.
federer, better overall coordination and body control.
all of this, incrementally so. it's a pretty close call on all fronts.
in terms of the broader discussion about speed, style of play makes this a tough call. with his style of play nadal has a lot more opportunities to show off than, for example, a player like stefan edberg. but if you lined the two of them up for a 100m dash, i might actually put some money down on stefan in his prime.
i think phau, chang and borg might be my picks for the 3 fastest guys i've seen play.