Sharebuilder costs less in every respect, including margin rates. Then there's the automatic investing...
Not sure how you figure this. Ameritrade has ~100 commission free ETFs, which include all the indexes (S&P, total market, etc.) that many Mustachians prefer. So basically, I use the Ameritrade service completely free. Hard to beat that.
If you're buying individual stocks on margin or whatever, Sharebuilder might offer some savings. Their automatic investing/reinvesting also sounded interesting, but when I looked into it, I didn't like the fact that it sounded as though you cannot control when your transactions go through. As in, the difference can be
days. I like to know with some certainty what trades will be priced at.