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It depends on the value of the SBD compared to the value of a Steem Token.
When they are trading at a 1:1 value then I would say take your awards in 100% SP. Currently the ratio is 1: 1.49, so if you take 50:50 your SBD will buy a little more SP, but it's another couple of steps because you've got to take you SBD and sell them, then take those profits and buy Steem tokens with it.
We are always in a position where we need to check what that ratio is in order to make the best buying decision...to get the most bang for your buck, so to speak. :)

@mystifact,
I just looked at this again. Right now the Steem Token is worth almost 2 dollars, so you'd be better to take 100% payouts in Steem power because the SBD is worth less at $1.49. You should always check this ratio to make the best decision about how you want to be rewarded. For months and at the time that I wrote this post, the SBD was trading higher than the Steem Token. In that senario it makes sense to to take a 50:50 split. Just check what this ratio is before you make a decision about how you want to accept your awards.
On another note, if you take your awards in 100% Steem Power, the funds are protected and you can withdrawl it in 13 equal weekly payouts. This is called Powering down. When you hold Steem Power it gives your upvote more weight, depending on how much steem power you are holding and what your reputation score is. This is because you are ultimately choosing to re-invest your funds into the platform which strengthens Steemit. Lots of people take their SBD and Steem and sell these on an exchange to buy other alternative coins...it just depends on what you need to do, versus what you want to do.
I hope this helps to clarify things a little more. I know that it is confusing.