Michelle,
6th grade is such a pivotal year math-wise, so our counsel in relation to that grade for math will vary based on the specific child's needs.

I don't mean to be evasive, but in my mind there isn't a one-size fits all answer.
One thing to consider is what the child has been using in math up to this point, and whether this has been successful. If this child has been successful with what he/she is using math-wise, then it is wise to just stay-the-course with the same math program.
If, on the other hand, the child has not had a good history with math, or is very frustrated with his/her current math, then it may be that the child should consider a switch to Singapore math.

To see whether a switch would be feasible, it would be wise to have the child take the Singapore placement test to see where he/she would have to begin in Singapore.

If the child ends up having to back way up in Singapore, then it may be too far back for the child to go. A good goal would be to finish 6A/6B by the end of grade 7 (or grade 8 at the latest), in order to be ready to begin Algebra I by grade 9 (at the latest). If this goal is not feasible with Singapore for this particular child, based on his/her placement test, then I would consider a different math path.
One other scenario to consider is the child who is not succeeding with math and seems to have huge holes or gaps in his/her math understanding and knowledge. This can be related to the child just not being mathy, or can be related to too much switching among math programs (thus losing continuity), or can be related to the child getting behind due to life's circumstances. In this scenario, we would usually recommend a math review type program for only a season (simply to help fill holes and help the child get up to grade level in math).

Once the child has been through the reviews, he/she would be ready to move back into a grade-level type program.
For those families who are not using Singapore for math, once their child has been through Pre-Algebra we would recommend heading into VideoText for high school math.

For families who have been using Singapore, we would recommend Discovering Mathematics (if they wish to continue on a more Singapore-like path for Pre-Algebra/Algebra), or Videotext (if they wish to take a more traditional math path for Pre-Algebra/Algebra).

Either will work.
Blessings,
Carrie