The answer is...both! My given name is Michael Steven Chavez. Following tradition in my mother's family, I was called "Steve" - or "Stevie" - from birth by members of my family, including extended family and family friends. Once I started kindergarten, things changed. My teacher, while calling the roll, called out, "Chavez, Michael." I dutifully raised my hand to indicate I was there and said, "Excuse, Mrs. Thompson, but even though my first name is Michael, everyone calls me by my middle name, Steve." Mrs. Thompson informed that since my first name was Michael, that is what I would be called. So, I got used to being Mike or Michael at school and Steve with family. When I started working, it was with people with whom I was friends at school, so thus I was Mike at work now, too.
When Kathy and I got married, our guests were uncertain if they were at the right wedding. My family saw gifts labeled for "Mike and Kathy" and our friends and Kathy's family saw gifts labeled for "Steve and Kathy." Fortunately, everyone stayed and helped us celebrate.
I'm occasionally asked if I self-identify as Mike or Steve. In my innermost thoughts, I'm Steve. Otherwise, I'm quite comfortable with either name. I used both on my blog because I use both on Facebook.