My answer wouldn't be biased.

Now coming to your question, I believe that hiring a personal writer always is a better option. You can call up your personal writer at any point of time and ask him to write, but you can't do the same with a writing service. Moreover, a personal writer will be more flexible. By this, I mean to say if you want an article on an urgent basis, then you can request him to do so. He would obviously comply with your request. But, you can't expect the same from a writing service. In case of a personal writer, you have someone who can be clearly (Visible) be held accountable for the work done.
However, in case of writing services, the articles are written by ghost writers and the person heading the team may lack knowledge about a particular article.
I am extremely sorry to all those who felt bad and were hurt by my comment. But, I just stated my personal viewpoint.
Hiring a personal writer has its share of disadvantages too. For instance, if you have few articles to be done on an urgent basis but your personal writer raises his hands at the end moment, stating his inability to write the article, then you will have no alternative option. However, a writing service has many ghost writers, so they will have at least one writer who can efficiently writing on the said topic. Apart from this, if your personal writer falls sick or has some personal problems, here again you will be in a fix. This may never happen with a writing service because they have many writers. If one writer falls sick, then there will be another writer who will finish your article.
There are various other pros and cons, but I suppose for the time being these points are sufficient. I wish you all the luck for your upcoming projects.
Cheers 
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