The vast majority of my MBA friends who were interested in S&T didn't have any trouble breaking into the field. Given the fact that you truly know that's what you want to do post-MBA, you should be in great shape, because you'll hustle, network and make the contacts needed. It used to be that S&T didn't necessarily hire a lot of MBAs. Things have changed. Given your financial background, I'd say you'll have a relatively easy time of it post-MBA.
I won't pretend to be a wall street guy, but given the little I do know, I'd be inclined the deriviatives ops group for the reasons you stated. Getting that intimate knowledge of the products, how their structured and how they're positioned would be great experience down the line in S&T, especially given the fact that derivatives are taking an increasing share of the product offerings. Besides getting some hedge fund exposure would be nice too and would potentially expand your opportunities should you want to trade (excuse the pun) in your S&T dreams for HF.