Hi James, welcome :).
Just from my opinion, if you already think you have a sound base in computers, then I would go with what you can learn most from, so in this case it sounds like graphic design. Doing courses can be expensive, so for me I would be working to get the most out of the course that I could. Plus this isn't to say that you can't complete papers in the other subject as well. The design will help back up your current knowledge in computing, and will help foster creative thinking and design.
Hope this helps, but really take a personal look at it from what you think your current skills are, and where you would like to expand, and go from there.