SBD is made to be valued at ~1 USD. Steem is not. So, if you exchange for USD, there is a clear winner. The market gets muddier though, because crazy people are actually paying more for SBD. Right now I am seeing the Steem Dollar trading at $1.43, which is higher than it's intrinsic value and thus a great trade to make. I would presume this is largely because outside investors are just buying and selling on the price points without any actual knowledge of the currency or Steemit platform and how it all works.

Also, I'm not so sure you can simply take SBD and exchange directly for USD, though I've not done it with Steem tokens either, so I admit I cannot speak to that so well.