I say one word. Always. Why? Because I think it flows better.
On the page, ‘copywriter’ gives the reader what they need to know in just one word. I find ‘copy writer’ a bit jarring. In fact, I always prefer one compound word to two words.
And when you’re speaking, or sounding it out in your head, the word ‘copywriter’ gives you four quick syllables that roll off the tongue with the emphasis on the first syllable. ‘Copy writer’ emphasises the first syllable of both words, or the first syllable of the second. Either way, I find it more awkward.
Both ‘copywriter’ and ‘copywriting’ are widely accepted. Even spell checks are familiar with the word.
But I’m willing to change my mind if a persuasive copy writer can convince me that they’re right.