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Holding Steem Dollar: give true interest.
Holding Steem Power is giving no true interest. You just get partly compensated for the 100% inflation of steem. From this 100% you get 90% back. In away with round about 5% ¨you¨ pay for curation rewards and 5% for post rewards. So if steem grows more then 10% each year and you hold your steem in steem power you win.

Yes, SP pays interest, but it's only to compensate for dilution. If you believe that STEEM price will go up, it's better to keep it in form of SP.

SP pays interest in the form of more STEEM which makes it more like a stock split or stock dividend. It mostly compensates for the very high inflation and (long-term) resulting declining price of individual STEEM token units; most of it is not actually earnings. SBD has a fixed (or at least stabilized) token value so the interest is there "real" interest (variable rate; currently 10%).

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