In my experience, it’s the first. It’s knowing what to write about. If you get the topic right, you can often get away with words that do an okay job.
Why is this?
Your potential customers will take a couple of seconds at most to assess whether to read what you have written. To reach this decision, they will look at your headline and, maybe, the first sentence.
It’s a bit like when you’re in a bookshop. You glance at a book’s front cover and, if that’s of interest, you check out the blurb on the back as well.
The topic you address in your marketing material has to grab the attention of your target audience. Your words don’t need to be flashy. But they must be relevant.
My Chambers Dictionary runs to over 1,850 pages and my Thesaurus has about 1,100. They are tools of my trade as a professional writer and are useful for fine-tuning.
But there’s little point focusing on precise wording if you’re writing about the wrong topic.