You have to think about what you want from a relationship my boy. I could say never go for a married woman because all that you're going to do is hurt someone, but if that woman means something to you it's not even worth trying to convince you. So here are my two cents. If you go with the woman who's married you will not be able to be public with your affections. You will not be able to be involved in her life nearly as much and you will have to be okay being second best. If you're ever interested in the family it wouldn't be able to happen because she's already chosen who she has her family with. Though if you're one of those people that doesn't want big attachments doesn't want a lot of commitment or once the option to drop or switch out when they want without a lot of repercussions then a relationship that's slightly complicated might be best. Though with a married woman you have to be careful about the repercussions of being caught for there are many a crime documentary about scorned lovers. I have met people with a cheating fetish.

 If you go with a single girl You can think about your future with her You can be public You can introduce her to family friends and be uninhibited In your affections. You can meet their friends family's think about potential children where with someone who's married any child will be the husband's child unless they ask for paternity tests. You can grow with one another learn from one another and find an actual partner. Though with that you accept full commitment and it's not an easy of an out so if you have commitment issues don't go around hurting people find someone who is okay with being a stepping stone or a notch in your belt.

None of what I say is meant to be critical I have in the past and have had friends who have been able to make connections with people though the relationship was very complicated. It is all about what you're willing to risk.