A users Steem Power (SP) and Reputation together can affect your & other users income, post visibility & their trustworthyness.
Whats more important to you will depend on what you hope to achieve on Steemit.
Lets break down each individual value
Steem Power (SP)
The Basics
SP is how much your upvote contributes to the rewards on a post/comment. It increases from post rewards and from powering up from Steem/Steem dollars. Its only reduced by powering down (converting back to steem).
If you earn/buy enough SP, you can shamelessly upvote your own posts and get post rewards off them! And you have the ability to support content you found useful/enjoyable without spending your own money!
Here is the catch
I know what your thinking, "If its that easy then why isn't steemit completely flooded with spam?". There are a few reasons
You have about 10 votes/day and at the highest single votes I've seen are worth around 6 SBD (from a SP of 57,000!). So you would have to either earn or invest 57,000 SP to gain $78 (at time of writing, 1 steem = 1.20110256 USD). Its a poor return on investment by any math.
Spamming posts is bad for the Steem community because people don't want to read it and will likely get you downvoted which will cause you to lose rewards & reputation, which brings me to my next heading
Reputation (rep)
The basics
Reputation is that number next to your name. Reputation effects your post visibility as well as a visual indicator to users of your trustworthyness. If you reputation goes below 0, your posts get hidden. You gain reputation from upvotes on your comments/posts by people with a higher reputation then you only. Likewise, you can only lose rep from downvotes by users higher then your rep.
Heres the catch
Because rep can only be gained by users higher in reputation then yourself it is a good measure of the general trustworthyness of the user, which may influence the users decision of what article to read (and trust). Unlike SP, Reputation cannot be increased using money.
If your goal on Steemit is to create a long term passive income then you will need to consider your reputation when posting comments/articles.
In Conclusion
- If your goal is to make a quick dollar or to make someone else a quick dollar, then focus on your SP
- If your goal is to gain a following, become trusted and be able to affect others reputation as you see fit, then focus on making quality content that will attract upvotes.
That said, there is no reason why you cant increase both & in the long term a good rep combined with good content/comments will increase views/upvotes which will increase SP! Have faith and keep working at it.
Note: Bolded words means how things are generally designed to work, the acceptions to the rules would be things like bots, vote blocks and communities