Itachi Handled opponents quick, get the job done to make his opponents use their reflexes.. This would mean his reflex is quite fast if he can set up for somethin as soon as you move or think of moving to only counter you if needed.
Sasuke has great reflexes because he's been trained in high lvl taijutsu since a buck. No wank I'd say they are about equal or Sasuke has a tad bit advantage.. (Itachi's sickness plays a role)