You have your answer already:
50/50 reward == 50 reward directly in SBD and 50 reward in USD(worth of SP)
But i would like to add one thing: If you plan to power up, do NOT select the "Power up 100%" option right now. That way you would actually being losing money.
You go for 50%-50% and THEN buy steem with your SD on the internal market. Because of the price difference, you actually got more SP if you do it that way, buying steem first using SD and then powering up, than going directly on 100%