My advice to anyone confused between radiography and radiotherapy is to get yourself some experience in the departments to see what you like most. Three years is a long time to be doing something when you're thinking you've made the wrong decision. The two jobs are completely different with the exception that they both use ionising radiation, they aren't really subjects that go hand in hand. If you train in one, you can't practice in another once you graduate. Put the effort in now so you know what you want to go for, and don't sit on the fence because chances are you'll always be wondering 'what if'.