I have worked at FIS and must say it is the best International School I have worked in. Its just a pity its in Germany
The students achieve high IB results and are well rounded students. Great sporting opportunities exist in the Highschool and the students are lovely. You may think the curriclum is slack, but this is because it is not prescibed as teachers are actually developing and extending your children's knowledge as an individual and not just by what a textbook tells them to teach. Check out the IB website for further information on this curriculum.
So in my opinion, I would definitely stick to the IB, MYP and PYP curriclum. I had students move from ISF to FIS and these children were the most uncreative and boring students I have ever met. They couldn't think on their own as they were used to Day 1, page 1, day 2 page 2 of the text book etc. and I am talking about Pre-school children. I personally don't agree with the SABIS system and believe that education is about the WHOLE child not just the results they receive in one exam.
Its up to you though and what you believe is best for your children. Goodluck with your decision.