If you use the market, it is actually running on the Steem blockchain directly. That actually doesn't use Blocktrades.

When you use the market though, it is technically not reducing the SBD debt, since the SBD is just traded with someone else in exchange for STEEM. When you do the 3.5 day conversion via your wallet, then it actually destroys the SBD in the process. It is a little riskier though. You can end up getting more or less STEEM that way - depending on the price fluctuation. Plus you have to wait :)

If you do 100% SP posts, then it doesn't generate any SBD (which is good) and it keeps STEEM out of circulation - which is also good :)