I asked this question before. And according to the answer I get:
If your post's payout is $100, as the author you get $75.

for 50/50 payout, you get an amount of STEEM equals to USD 37.5 ,
plus 37.5 SBD (this part alone will worth about USD 70 at this moment). So you actually get more than $75 in total.

for 100% pwer up, you get an amount of STEEM equals to USD 75 according to market price.

So, you should choose 50/50 when SBD worth more than $1, and 100%power up if SBD worth less than $1.