There are advantages & disadvantages to both.
Married men won't brag about it (discretion), are less likely to bring an STD into your life and they mostly desire to have a strong & lasting sexual experience that is missing in their marriage. However, their availibility is usually limited and takes advance planning on eveyone's part. You do have to be careful of not getting into a marriage wrecker situation with a pissed off wife or having the dude fall in love with you.
Single men usually just want to fuck and are easier to schedule. But, you don't know where his dick has been lately and if he's got a big mouth. They also tend to no-show more often.
One of the best situation we've been in was a married black man who traveled to our city on a very regular basis for business who we met on a dating site. The three of us had wonderful sexual experiences including being able going out together for dinner and dancing before the sex. Having that social interaction just makes everyone so much hornier.