I finally found an actual sample of Singapore... wow!! It confused the heck out of me!
It was just a few of various pages, and they all had word problems on them and seemed just... I don't know... kind of strange to me, I guess. I'm thinking that's not a good sign.
So I printed off a couple of sample pages of MM, and had DS look over them and do a few problems. Then, just bc I stumbled across it, I let him do a sample of Teaching Textbooks. He really liked both of those. After we discussed them at length, he said that since the MM was so much cheaper he'd be happy to use that one. Such a considerate boy sometimes!
So I guess I'm off to order MM -- please cross your fingers that this works well for us!!!
If it doesn't (and probably in a couple more years either way), I'll try the Teaching Textbooks.
Thanks for the advice everyone!!!! We both appreciate it so very much!!