Great question. I did an article on the 3 currencies here https://steemit.com/steem/@bitdollar/steemit-s-3-currencies-explained-and-how-to-maximize-your-returns
If you are planning to grow and do well in the community, then I suggest building up your Steem Power. This will give you much more influence and build up your profile big time. As well as a good long term investment. Steem I would just trade in and out for short term profits. Steem Dollars is good to hold as you can exchange for whatever you need and it earns interests while it sits in your wallet.
All depends on your goals though.
Goodluck!
How does the Steem Power -> Steem conversion rate work? From what I understand the conversions from powering down take place once every two weeks in equal increments for two years. Does each conversion over the two years use the rate when you initiate the power down? Or does it adjust to the current rate at each payment? For that matter, what is the conversion rate from Steem Power to Steem?
Not sure on the conversion but it would be the adjusted rate, which should be better than the initial.