I've tended to work with repeat clients, honestly I dread working with new clients, you just never know what they want until after you've given it to them, and then find out their requirements are different from the instructions they gave.
Having been a writer for many years, around 90% of my clients gave me repeat business over an extended period of time. Many have also become friends.
The pros of working with repeat clients are that you get to know them, you get to know the context they work in, and where your work is going to appear. You also get much better references and more word of mouth referral business from them.
On the downside, repeat customers often expect more frequent discounts. There are a lot of other financial problems that can arise from repeat business, for example delays in payment (you're just another supplier), to tax problems if you work too closely with a single client (difference between being a contractor vs employee)