Basically there is just one cryptocurrency: Steem. Steem Power and Steem dollar are just financial derivatives that can be changed back to Steem, although SP has also voting power.
From a financial point, SP is for those who want to protect their investment from (Steem) inflation and SD is for those who want to have price stable cryptocurrency against fiat currency (USD in this case).
Dollars and Power. That's all there really is.
So what trades on other exchanges like Bittrex?