Investing in STEEM Power is investing STEEM. The way our blockchain works is that the STEEM tokens secure bandwidth and enable stake weighted rewards when powered up. When in the powered up mode, you are essentially locking your tokens for a minimum of 13 weeks. I often tell people to think of SBD as an SMT that is supposed to loosely be worth $1 USD, but the peg can be broken or manipulated.
If you want to invest in STEEM and use the platform, it makes a lot of sense to power up as there are many benefits gained from holding the STEEM in a powered up capacity. You can vote on posts and distribute rewards and it also secures your bandwidth to allow you to post or interact on the blockchain. If you have no plans to use the platform and simply want to invest speculatively, then holding liquid STEEM is your quickest option for resell, but arguably less profitable since you can't gain the benefits of staking.