They rank these services by Min / Max ROI, which means that your purchase can compound up to 148.75% return on investment, as SmartSteem says on their promotion side:
'Fixed 10% ROI. (5% Bonus for limited time) Ratio is increasing linear from 119% to 148.75% (at 25% curation).'
What does this mean for you? Should you acquire as many paid votes with up to nearly 1.5x returns in the first few minutes when your post is worth 100% of the voting proceeds back to you, you can simply take some funds you have and invest them as STEEM or SBD into your posts, then wait a week to get 1.5x! your money back.
This should give anyone the motivation to write.. so write!