SBD is supposed to be a stable token, pegged to be equal to 1USD of STEEM, so it should hold its value at that, but doesn't quite do it all the time. The platform is designed that the SBD shouldn't equate to more than 10% of the total STEEM market cap, as it is a convertable debt token.
When the SBD/STEEM debt ratio gets too high, post rewards change from being in SBD and SP to STEEM and SP, to boost the amount of STEEM being produced, and halt SBD production.

Whethet to hold STEEM or SBD depends on whether you think STEEM will be going up or down in value, and whether you want to power up into SP for more influence (and reward) on the platform.

Hopefully a few words in there makes some sense...