harrass or farm?

Im trying to get better but my CS always seems to be low, im relatively sure that it's mainly because i focus on harrassment of the enemy champion at the expense of CS, should i do that or should i focus on CS and take what harassment i am able to dish out without endangering CS? also how do the carries always seem to have 400 cs when they take their opponents towers and always have minion waves pushed, it seems like they should have less CS than they end up with?