I think some of the others may be able to give better details.
Normally something like multi-vitamin brands are similar, but definitely large differences can be seen when comparing cheaper brands to the better brands. This has to do with how your body intakes each of the ingredients. Brands will use different salts or pro-vitamins, your body uptakes each differently, ie. your body may have to expend more energy to use a certain salt than another, or at a certain concentration your body may just flat out not uptake anymore.
I'm not going to go find the science articles, but I'm sure you can find them with google