The power dynamic between Antonio and Shylock seems to change over the course of the play. In the start Antonio would certainly have a far higher social standing and would therefore be in a position of power over Shylock. As a successful Venetian merchant he would stand well above a Jewish moneylender.
Things change slightly when Antonio needs to borrow money on his friend's behalf. Now they are on a more even footing. The two are engaged in a business deal together. Antonio even agrees to give his life if things go poorly.
Some might say that towards the end of the play Shylock is in a position of power over Antonio. Since the merchant cannot repay him, the moneylender is now in a position to take the old man's life. I would argue this is not the case. As we see from Portia's skilled rescue, the society they live in will not allow a man such as Shylock to have control of a man such as Antonio. If the business deal were to go well for Antonio he would get want he wanted, but if things go poorly for him, he can always go back on his deal, either by trickery or by trade.
This demonstrates that no matter the business advantages he may have, Skylock will never be able to assert true power over his Christian neighbors.