There's no simple math that gives an answer for what's best here. Generally, you get more attention if your post ends up higher on hot or trending, but that's a factor of not only how much SP is behind your post, but also how much is behind other posts. Ideally what you want is to put your post somewhere in the top 5, top 10, etc. If you have tons of money and plan on posting a lot (or promoting some content you like), then you may just want to buy SP. If you don't want to buy enough SP to achieve a top 10 position, you can buy a delegation and get more leverage (on a shorter time frame) to get your posts ranked higher. Or if you just want to promote a few posts, you can just buy a vote on a bidbot and get very high, extremely short-term leverage.
Each strategy has different benefits, and there's never going to be a clear winner in all cases, I think, as the best choice is based on your circumstances and the behavior of other people.
With a limited budget you should always aim for the most cost effective strategy. Regardless.. I still believe that delegation is highly overpriced.
SP has lower value then liquid steem due to the time restraint of powering down and lack of liquidity that imo isnt offset by the curation earning potential due to volatile market. Bought delegation offers even less value do to having to be returned to the delegator after set time.
Bought delegation needs to offer a positive ROI after the 3 month date comes to pass.
Otherwise its a foolish investment that impovereshes you. Thus, eventually taking away your power to have any effect on the reward distribution or being a part of the social dynamics here.
On the other hand, Powering up, Using bots(rather upvote services) creates a long term certainty of retaining influence, having a potential to empower others long term and of you being a "factor" on the steem blockchain.