SBD, is considered a backed currency by steem indicating the pegged value of 1.00 however it is not that case currently! Steem is the currency of full liquidity meaning, you can send it anywhere via exchanges etc.
SteemPower is powered up from Steem it self, which in return gives you a higher voting power, to upvote people with a higher amount as you power up the steem itself. The downside you can say about powering up, is that it takes 13 Weeks in order to get your money to be liquid again. However if your on the platform daily and beleive in the platform, that is not a down side! Took me like a month and a half to really wrap my head around it.