1, Do the 9 cross points have a big difference compare to 600d?
It depends what you are shooting. What it will help do is lock focus faster, and its useful for shots where things are moving (people, cars, animals, etc), since it has more points to differentiate what is what.
If you were using a 600D right now, you'd probably notice it gets focus correct a little more often; but it's not what I would call a big difference (the 600D v 7D or 5dIII AF would be a BIG difference). But, as features go, it's one of the more useful areas to have an upgrade in, since the AF is one of the biggest elements that decides whether or not you get the shot.