Long story short :

If your post earns 10 steem$ :

2.5 goes to curators (people who upvoted the content)

if you go for 100% you divide the 7.5$ by the value of steem (~3$)

7.5/3 = 2.5 steem power.

if you go for 50/50 :

50% gets converted to steem power : you earn 1.25 steem power
50% is converted to sbd : 3.5 sbd, which can then be converted to steem at a 1:1 ratio (it often hovers around that value) so that's 3.5 steem which can be powered up.

So in the end you earn 4.75 steem power if you do 50/50 vs 2.5 if you did 100%