For those who want to take full advantage of Steem power (SP) without participating in the Steem blockchain at all, there are bots which can help in two ways: SP delegation and Vote-selling.
I've compared the the SP delegation profitability to 
Vote-selling in Minnowbooster
First of all, the payout. You get 85% of all the sold votes they cast for you. Plus curation rewards which are coming from the posts payout which is totally unpredictable as the bot can cast votes for all the vote buyers.
Minnowbooster does have a referral link for new member recruitment which I did not expect at first. However, I failed to locate any detail on how they handle the referral bonus to referrers. Is the referrer credited if the recruited filled a lease order, vote-selling or bot delegation? I tried a several searches on Google and Steemit, including a thorough scan at their FAQ page, all to no avail.
Please enlighten me if you know any information on this.
Vote-selling in Smartsteem
Vote seller receives 85% of the profit which is same as Minnowbooster. However, Smartsteem provides a really convenient Revenue Calculator which allows users to get a rough idea how much they are going to make. '
While you still get to receive the additional Curation reward that incurred during vote-selling process, technically it is impossible to predict how much you are going to make out of this extra bonus.
With the exclusive SmartScore Stars system, I would say one is likely to score more handsome rewards with Smartsteem.
Under user Settings you will find this section of Vote User Selection where you can decide who you will sell your vote to. With SmartScore Stars selected, your vote will only cast to the whitelisted users which you can review in this list. All the whitelisted user is hand-picked by the team and I have picked several users for quality check, almost all of them are quality content creators I would say or you can kindly check the whitelist out yourselves. The team did not reveal their star rating standard but they did a fair job on it nonetheless.
We know the first rule of Steem, good content usually get rewarded more.
So I think it is safe to say that vote-sellers can get higher Curation reward via Smartsteem provided they sell votes to the SmartScore Stars users.
Unlike Minnowbooster, Smartsteem makes everything clear on the referrals bonus that referrers will receive 0.5% profit of every sold vote by the referrals. And I'm receiving referral bonus from my recruitment.
However I cannot tell which service is more superior in this aspect unless Minnowbooster makes a clear declaration like Smartsteem did. Nor did I ever referred anyone to Minnowbooster.
Note: One can only receive referral bonus by vote-selling in Smartsteem. 
Conclusion
Both services provide 85% profit for vote-selling.
Curation rewards are unpredictable but Smartsteem tends to be the winner with their killer SmartScore Stars system.
Both services carry referral system while Minnowbooster remains totally unclear and Smartsteem provides a solid 0.5% profit.
Minnowbooster seems like doesn't care much about vote-selling service as their main focus is in the well-known SP Lease Market. At the same time, Smartsteem treats vote-selling as the core business that they build some complimentary stuff like Whitelist and Vote Selection around it.
Smartsteem is much more preferable in vote-selling selection.
Please use my referral link to sign up Smartsteem if you found this article useful: https://smartsteem.com?r=fr3eze