Doing the 50/50 split and using those SBD to buy the Steem will actually get you more than if you'd just taken the 100% SP option. This only works out when the SD is worth more than $1, because the 50/50 split pays you as if it was at $1. Last night the SBD dropped to $1.25, so it's becoming less of a benefit. However, with it at that price, you'd actually be getting an extra 25% 'buying power' with the SBD you'd get over having it through 100% SP.