Admittedly, this isn't going to cover your question but, but here is my take on what I know:
1. Dreambox is more "game" driven and visual. My DS6 likes it because it's more like playing than work. I don't think it allows for much skipping around - that is, according to his teacher, you can't move from one concept until you have mastered a previous concept. This is a blessing and a curse for gifted kids - (pro is fewer knowledge gaps, con unable to explore new math passions).
2. We've used Khan Academy a few times. It's like a YouTube classroom smartboard instruction. I like it as a supplement if my son is interested in something. It's great for me - a non-gifted parent of an HG child. I haven't thought of exponents in 20 years and didn't imagine I would have to reeducate myself when my son started the 1st grade. I figured I had a few more years- so, Khan Academy can be helpful to us regular folk trying to explain math to our more intelligent children.
3. IXL- my opinion- computer based Kumon. If you can get your child to do it, it's excellent for math drills. Mine doesnt like it either. I am not skeptical of math drills (many are)- but it helped my DS develop better fluency and become less frustrated when he automatically could calculate the component parts of a harder problem (e.g. Knowing the multiplication table "cold" sure helps stem frustration with long multiplication and division was almost an afterthought once he got that
table).
4. I'm not too familiar - we bought a workbook because we're considering a school change with the new school using Singapore. Based on what I see, I love it- especially the challenge math section at the back. In my opinion, it's more visual with a stronger emphasis on analysis than some other curriculums I've seen. I want my DS to have an arsenal for solving a math equation, which calls for strong analytical skills and mastery. Singapore math fits the bill for me.
