The only scenario in which I see a good option the power up 100%, is when the price of the steem and the SBD is less than 1 USD and its parity is less than 1
Suppose that the steem and the sbd have a price of 0.3 USD and 0.29 USD
Power up 100%
0.25 * 75% = 0.1875 / 0.3 = 0.625 SP
50/50
0.25 * 75% = 0.1875 * 50% = 0.09375
You receive
~ 0.09375 SBD
and
0.09375 / 0.3 ~ = 0.3125SP
The SBD / steem parity for this example is 0.97
0.09375 * 0.97 = 0.0909375 steems + 0.3125 sp = 0.4034375 steems
In this scenario is better power up 100%
0.625 Steems for power up 100% and 0.403 Steems 50/50