Depends on what you find rewarding, I guess. I think being an oncologist would be rough...get more attached to patients and see the effects of the disease/drugs more closely. As a pharmacist, I feel like I'm able to help these patients be cured or live the best life possible until the disease wins, without the close relationship that would tear me up inside. Lessons learned the hard way, trust me.
Clinical inpatient oncology (not including BMT, I've never done that)--lots of patient monitoring (not just oncologic but also other co-morbid conditions), counseling on chemo, checking/double checking inpatient chemo regimens etc. People with cancer still have other diseases too, so you have to know more than just oncology.
Outpatient infusion oncology--checking of chemo mixtures, checking labs and dose verification before mixing, chemo counseling with patients, drug info questions from providers, premed tweaking, etc. Many outpatient centers are doing oral chemo counseling clinics now, too.
Unfortunately a cure for cancer is a long way off; fortunately for oncology pharmacists that means job security.