Nolva and Clomid are almost identical, in both result and side effects. Both are estrogens and give estrogenic side effects. As nyd66 said, they are SERMS, meaning they are selective in the estrigens they "replace" in the body. But you never know how they'll react. In some people, they remove the bad estrones, in others, they simply add excess estrogen.
Nolva will block estrogen at the breast site, so I guess it makes it a viable anti gyno drug. But arimidex and non aromatizing steroids (or reasonable dosages) will stop the problem before it occurs.
Why people still use them is a complete mystery to me. I've been saying this for over a decade now and every day I have to read about people adding them to their PCT. You might as well use cloraform as an anesthesia or leeches to remove toxins. They're just chancy, outdated methods. But old habits die hard.