I know that copy writing doesn't have to be Dickens material but not everyone is meant to be a writter.
I would invest the money in content first and theme second. But the answer to the question theme vs content isn't always black and white. Depending on the niche, you may need a quality theme for the beginning; so that, your site is taken seriously. It's also possible to start with a quality theme and build out the content over time.
But look at it this way ... Let us say that you have a restaurant with the right ambience. However, the food is mediocre at best. You may end up making less money than a guy with great food in a low budget storefront.
Content is king because, Google consider & deal content as king. The next obvious question which will arise in our mind is that why it so? Simple because visitors like content. Theme is also has a significances but relatively content is superior.
Web design or theme and content works hand in hand to generate traffic and improve trust and loyalty of your audience. A well written content can attract people to participate in the conversation while the theme or web layout will can help to keep your audience feeling free to browse and stay longer on your site.
As far as SEO, themes "can" make a huge difference. For example a theme that supports custom excerpts is MUCH better than a theme that displays the entire post on the home page, category pages, archives pages, and tags pages. The latter creates tons of duplicate content. The former does not.
You can make up for some SEO shortcomings of your theme using something like All-in-One SEO Pack... but even All-in-One cannot fix the above problem.
Themes AND content play a large role in how users view your site... which over time... can affect whether or not people link to you naturally. A site with great content and whose theme gives it a professional look and feel WILL attract more natural links than a site with the exact same content using some crappy looking theme.
Content vs Themes, I think you need a bit of both. I can land on a pretty site but if it has no useful info for me then I'll leave, but if it has good content despite an ugly site I may stay and read, if it has a bit of both then it is a winner! In the end, write useful information for the reader, worry about SEO last as you can incorporate this at the end.
I focus on the content of a blog first, then to the theme. In fact, I usually change my themes a few times during the blogs life until I find one I like. The content will always be there no matter what style you pick ;)