It all depends if you believe in the site for the longterm, or think it will fall apart. Powering up is a great way to gain attention, because people follow the power for votes. If you believe longterm, then the price of steem is cheap right now so you should be powering up as much as posible, Plus the rewards are higher right now then they will be in the future.
If you believe the Steem price will tank, then holding SBD is a better option because you will have much more buying power in the near future if you decide to power up then.
The big drawback to SP tho is the 102 payments over 2 years. The site could dissapear by then and leave you will worthless steem that could have been real cash at 1point.
If you think steem is your calling and you plan on being here in the future, buying all the SP you can buy is the best choice. You gain interest on SP while providing yourself an increasing amount of power.
The choice is in your hands