Check out these two encyclopedias: International Encyclopedia of Communication (https://bit.ly/2PY5ixA) and the International Encyclopedia of Linguistics – which includes an entry on sociolinguistics (https://bit.ly/2pPWgYU). They’ll give you an idea of the range of areas of study in each discipline. Why don’t you make an appointment with a faculty member in the Communication Department, and one who teaches courses in linguistics, to chat about their graduate school experiences. And you can explore the course offerings/requirements, and faculty profiles (and publication history) in both communication and linguistics graduate programs. Perhaps these next steps will help cement your interest in one area over the other. Good luck with your next adventure!