I was unable to keep my vitamins down with my first preg, and my doc told me that I could take 2 flinstones everyday, and that would be the same. So from then on, that is what I took with all of my pregnancies.
I'm using flintstones. A vitamin is a vitamin. I use the "Complete" version of them and it has enough folic acid and iron, etc. so I figured it would be ok.
I think that the only difference might be that the ones from the doctor often have a stool softener in it, since many people have trouble with constipation while pregnant/taking the vitamins. I just use the prenatal vitamins from the grocery store.