If you are investing in steemit then you can better go for steem power because steem power you will get return easily after 3 months and you can also build a high profile steemit account with the help of steemit and can earn huge amount after having good blogs.
If you only invest in steem dollar you can pay to several bots you can use them to get boost of your post very easy and this step is at risk because the amount you get in interest in not fix it will be more or less also.
So the better is you go with steem power and can withdraw after 3 months whatever the price of steem will be on that day.