I suggest playing through the whole campaign and then make the decision. You should know the mission objective, so if it is timed, get in, do the job, get out. If not, use a different tactic. Play Under Siege, then play Diala's mission again (ignore your first attempt)
First time I played the campaign, we were not told of the time limit. I felt really cheated. Come to find out, if we knew the whole objective, we would have played it differently. Second time around we succeeded, barely, by blitzing our objective and not eliminate our enemies.