I haven't read all the replies yet, but given that there are protective properties to breastmilk, I would hazzard a guess that bm would still be better from a toxicity point of view.
Our food supply is a toxic mess. That is not an exaggeration either. So it makes sense to me that a number of those toxins would be passed through bm, BUT breastmilk is amazing stuff with many different jobs besides simply providing nutrients. ie, one thing it does is train the immune system (70% of which scientists now believe is in our digestion) how to process and eliminate toxins.
I think it would be naive to think that many of the same toxins wont' make their way to our babies anyway (sadly) through our water supply, air, food supply and even in the ingredients that comprise formula.