The top tier of content writing services is the Content Marketing Agency, where you'll find professional writers that often create the best content that competes at a high level. This group is where my company falls into, and the group is a bit smaller and less common, with varying levels of detail and with varying degrees of service. Nearly any business with a budget for content can find a full-service content management agency to handle it. It's not a great option if you're running on a shoestring budget, but if you have a bit of money to invest in good content, it has the greatest chances of success. You'll get a much more individualized experience, you'll rarely have to do proofreading passes for errors or fluff, and you'll have more than articles written for you - you'll have a complete content strategy with expert writers.