If SBD rises up, it allows us to buy more STEEM right?
Thats correct
Which creates an opportunity to have higher SP as well (if we power up)?
Correct
Or they really prefer to cash out due to the higher equivalent for a fiat money?
This depends on if you think Steem will rise or fall in value compared to the dollar. If you think its worth investing in Steem buy Steem and Steem Power, if not you can cash out.
Thank you for this @eroche!