Generally there is a certain amount of overlapping. But sociolinguistics would be a subdiscipline of linguistics, whereas sociology of language belongs to sociology. Disciplinary origins of your inquiry will let you tend to either of the two options.

In practice, the boundaries between the two become very hard to distinguish. For instance, in a study done on complimenting in a particular language, one has to both refer to linguistic features and sociological factors to provide a clear picture of complimenting. Both complement each other.