I have learned this the hard way.
If you book ES and you want to change your date you do not pay always $50, you pay whatever penalty category you are in. i.e. I was within 32 days of sailing and needed to push my sailing back 1 week and it cost me 50% of my booking rate!
So, if you are absoltely positive nothing in your life is going to change then book ES, otherwise you may want to check out the other rate. I'd have saved money by going with FunSaver, but life and learn.