I'm a radiation Therapist, so I might help.
1. Outlook.
It's a great field, with the baby boomers getting older and cancer incidence increasing, you can be assured there will be jobs. There are new clinics opening up here in Canada, and I am sure in the USA also since you have a larger population. However, right now some schools are pumping out too many radiation therapist in the USA. You have to be willing to relocate in some areas from what I hear down there. Good thing about the field is that it is a 9-5 kind of job with weekend emergency cases, but you sign up for that. One of the only health care fields are are designed this way.
2. Radiation therapist are the ones who deliver the treatment and see the patiente everyday. They also provide guidance about management of side effects and support the patient during their treatments. Most patients are treated from 1 day up to 8+ weeks, depending on the cancer and type of treatment. So you develop a great relationship with the patients since you see them everyday.
Dosimetrists are the ones who PLAN the treatment with computer systems. They figure out beams, energy of radiation, making sure the prescription is delivered and to not exceed organ tolerances. They have little patient contact and mostly work with the physics group. It's more stressful because what type of plan you come up with, will affect the patient with respect to curing them and the severity of the side effects.
In Canada (where I am), we Therapists are dosimetrists and we do both. We are trained in both aspects. In the USA, the two are seperated, but most dosimetrists are former therapists. It is better that you are a therapist first because you learn what is possible on the units. It will make you that much better. But there are a few dosimetry programs in the USA. In Canada, there is no seperation of the two fields. As a therapist, I am also a Dosimetrist, though new graduates start off as Therapist, and with few years of experience, can move up to dosimetry.
3. In the USA, you don't need a BSc, there are associate degrees. However, if you want to become a lead or chief therapist in the future, get the BSc, it give you more options. But it is not a must to work, everyone takes the ARRT exam. In Canada, mostly degree programs and we switching to it.