If you are looking to invest long-term, changing Steem into Steem Power is the way to go. As @etcmike did a good job explaining in this article, you will get more Steem if you are holding Steem Power. If you just held on to the liquid Steem (without powering up to SP), you would not get the additional Steem added.

Also, one other thing to add. I've found that converting my SBD into Steem on the Steemit exchange ("Buy or Sell" in the wallet), then powering up right away gives me a better value than waiting the 1 week for it to convert. It could play out either way, but with the way the price has been behaving recently, the quick conversion has been paying off more.