If a state constitution and the US Constitution conflict which one prevails?

Always the US Constitution. The USC was developed before any state laws or state constitutions and in all court hearing you plead your case the United States Supreme court. States are allowed to interpret laws, though the US Supreme Court may overturn a decision. Once a law is formed all states must abide. That is why there was such turmoil when George Bush tried to ban Gay marriage. If he were successful they no state would hve the right to perform or recognize gay marriage. That being said if there is no law, a state does have the option to create and inforce laws to help keep it's citizens safe.