Steem dolar is great invention because it is always same as regular dollar period. You know how much its worth anytime.
If u like trading then you are better in steem so you can dig your way up through fluctuations of steem value and earn more dollars.
If u are not trader and not willing to take risks you just sit in your steem dolars and earn your self 10% interest (yearly if i'm not mistaken).
And if u buy your steem with steemdollars better do something with it don't let it stand there for to long. Either work on market (riding waves) or power up to get 90% interst rate in steempower (but there is catch that you can't get out fast) or trade those steems for more steemdolars or btc.