We did RightStart A and about half of B. It was teacher intensive, so much so that we weren't getting math done. I LOVE the foundation it gave my daughter though, and glad we were able to stick with it at least that long. It is making life so much easier now... During the time I slacked on actively teaching math, we used Math Mammoth, we used it all last fall. Both my daughter and I did it willingly, but it didn't bring either of us any math LOVE. No passion there. I suppose most of you don't equate math with passion, lol, but I happen to love math. I chalk up our lackluster feelings towards Math Mammoth to my daughter's learning style and my teaching style. Yeah, RS was too much for me, but MM was too little, hehe. We wanted something more social, more colorful, more energetic, and some hands-on.
Then we started Singapore. We both fell in love! My daughter, for the first time in her life, is excited about math, so much so that she talks about advanced concepts in everyday conversations, things we've never even covered yet (2-digit addition, multiplication, negative numbers). I'm stunned that this change came about after the switch to Singapore. It offers challenge to her in ways that other math never did - if you use the Intensive Practice and Challenging Word Problem books. It's exactly what we needed.
That's just our case, both are great programs, it's just that Singapore brought out more math love for us.
I purchased all of Math Mammoth Light Blue Grades 1-6, so if we need to supplement or go on travels, we'll always have a good math program to fall back on.