Well, SBD has been growing at a faster pace than STEEM. For example, right now SBD is $1.89...but looking at the past, it's gone up to $18 and STEEM was at $8. This means when you convert your SBD to STEEM you get a lot more STEEM which you can use to power up your SteemPower. That to me is my biggest objective...get more SP!
Of course, this is not financial advice...blah blah blah...trade at your own risk.
SBD goes up quicker because it has much less circulating supply in comparison so it goes up ahead of steem most of the time. but when there is a gradual decline over a period of time SBD will go below steem as there is no point in keeping SBD for longer periods it does not earn anything. but if you are keeping an eye on these, you can convert back and forth to increase your holding.