A fiancé visa, of course, is only meant for people that are not married. An immigrant visa is only meant for people that are married in this context. They take roughly about the same amount of time to bring the person to the United States. However, the fiancé visa has the additional step of adjustment of status, whereby after marriage, the foreign national must apply for their green card. It depends on the circumstances, but it would appear that the immigrant visa would be the faster route, as it has one less step.