I'm going to look at three cases
Case 1: Investment in Steem more than doubling every year(Optimistic)
In this case, you can't really go wrong, invest to make money, invest and contribute to make more. We need it to double due to Steem's inflation rate been 100% a year.
Now a lot of people think this is a given, as we all know so many people interested in steemit and, barring some disaster of course, steemit users will double each year. But new users on Steemit doesn't mean new investors in Steem. A lot of us early users are from the crypto world, and I reckon the 'investment to new user rate' is much higher now than it will be when steemit goes mainstream.
Case 2: Steem Investment about doubles each year (Achievable)
OK so now investing and only investing is a wash so we can talk about contributing. Now everything I say about contributing applies to the other cases as well.
That 100% inflation I mentioned, well 90% of that new Steem is earmarked for rewards. I've heard some people say 'turn your Steem into Steem Power and you don't need to worry about the inflation', it helps but it is not fully true. Here are the rewards you can get
- Interest
Check out this cool site by 
You can see how much interest your SP is making just sitting there, mine is about 5% APR, so not enough to combat the 100% inflation. This is my main point of this post Static investment will struggle to see a profit
- Posting/commenting
Something on your mind? Got an idea? Post it, if you pick the right community someone will be interested. The lion share of that 100% inflation is given to these posters and commenters.
- Voting
That dollar amount you see below a trending post doesn't all go to the author. 25% goes to the curators (the voters). And this distribution is weighted so that, if you vote early on a post, your reward will be bigger.
- Mining
I believe 10% of the newly minted Steem goes to these engines that keep it all running, but am no expert on this aspect so......
Case 3: Steemit fails
Bah no chance, this site is great. But if it does? Well it is a startup so of course you need to consider steemit failing as possible or maybe even likely, Only invest what you can afford to lose
For more info check out the Steem white paper
TLDR
Contribute to increase profit whether you invest or not.