I think the big difference is that when you hold bitcoin, you are hoping that more people buy bitcoin so that you can sell it at a higher price than what you bought it at.
With Steem Power, you can buy it because you want to use it. If you're an advertiser or creating a new product and want to give it attention, you can use Steem Power for that.
Bitcoin has nothing really backing it's value. Steem Power is backed by content and an ability to bring eyeballs to content.
RE: Why Should an Investor Prefer Steem/Steem Power over Bitcoin?