GIS is a hybrid of Geography and Information technology. It exist in both disciplines but the research interests are different in both. The basic concepts of GIS about the data types, development issues and analytical methods all are from Geography. While the software side (all tools and software interface) is from Information Technology.
Now it depends what you have already studied, like basic Geographic concepts (about location, maps and Spatial analysis) or basic concepts of Information Technology.
If you are interested in software development and programming then your choice must be Information Technology. If you are more interested in Analysis and Geographic research then Geography will be your right discipline. It all depends upon your interest, you can choose any of them as per your aptitude.