Here's my (non-expert) take on the comment by Hive account@hilarski

The main difference is actually between selling SBD Vs using the 'Convert to Steem' option on the SBD balance drop down.
This also depends on the current exchange rate of SBD to steem etc. Selling SBD on Bittrex or the internal market is usually advisable due to the price increase of SBD which should be pegged at around 1USD but is currently higher due to what I will just call 'Market forces'
^{[as it has to do with the exchange, the current steem price and the mechanism (effectively a debt mechanism against the steem price) by which SBD is valued.]}

The most reliable long term strategy is probably just looking at the current bid price on an exchange (like bittrex) Vs the internal market keeping fees in mind.