We switched from using Saxon last year to Singapore this year. My son completed Saxon 5/4 last year, and did fine with it...but he thought it was so tedious and boring. We had the hardback textbook and he had to copy each problem; we struggled with dawdling as well as with copying problems incorrectly.
I decided to give Singapore a try, and we're loving it so far! I had each child take the placement test. My ds place in 3B and dd in 2A. My dd did 1A & 1B for 1st grade, used Horizons 2 for 2nd grade and did amazingly well, but essentially 'missed' a year with Singapore and ended up going into the same level this year (3rd) she would have if we'd stuck with it for 2nd.
My son placed in 3b, and he's in 5th grade. However, Singapore is very different from other programs and is advanced, so I wasn't really concerned with placing him in a 'lower' level that it would seem he should be in. He has been doing 2 lessons a day and will likely be into level 4 by January, so starting 'lower' has been good for him. It has allowed him to get used to the 'new' way of thinking and doing problems, so when he does get to the higher level he can concentrate on the problems instead of still trying to get through the learning-curve portion.
My point is (sorry it's taken so long!) that the placement test is an important tool for getting your chld in the correct level. Your dd may place higher than my ds did, and that's great! But I just wanted to assure you that if she doesn't not to worry, because Singapore is more advanced and completely different in many ways from Saxon. My dh, an engineer, thought both programs were good, but really likes the way Singapore is laid out and thought our ds would learn better with it.
Also, dd is using 2A right now, and is doing 2-3 pages a day, even though the guide has less scheduled. We may change that as she gets further into the book, but right now she is easily getting the problems done. I think there were so many problems per page in Horizons, the layout of Singapore seems easy to her. It's a good thing!
Good luck with your decision...it's sometimes tough to know what to choose. I know others here will be along to help as well.