My understanding is that Hem/Onc docs are in greater demand than there is supply. Thus it is not unusual to hear of people making 300 out of fellowship. There is talk that reimbursments on chemotherapy could affect this salary, but its unclear to me how reimbursment issues would alter the supply of doctors in Hem/Onc, except in a negative way.
Moreover, while it is nice that Hem/Onc garners nice wages at the moment, in my opinion, it is the last field where you would want to take money as a major motivating factor. Such thinking seems fraught with problems at the outset. Having said that, as a specialist, you will earn specialist wages regardless of changes in chemo reimbursment. Before the recognized shortage, Hem/Onc specialists were making around 200.
As for competitiveness, Hem/Onc is one of the less competitive subspecialties in medicine. This may be because people have difficulty working with the terminally ill. While you may actually cure patients as a pediatric or gyn oncologist, much of the work in medical oncology is palliative. Nonetheless, currently about half of the fellowships in medical oncology are taken by FMGs. It is not hard to match into a fellowship. If you are talking about training at Sloan Kettering, Dana Farber or MD Anderson, however, that's another story.
As for whether the field is depressing, you obviously have to find this out for yourself. You may be surprised to discover that some cancer patients can be truly inspirational in the way they deal with illness. Whether this can outweigh the challenge of countless difficult conversations is obviously a very personal matter. During my limited exposure, I have personally found it more difficult to talk to families than patients themselves about cancer.
If you are interested in oncology, look into pediatric and gyn oncology as well. These are more competitive, especially gyn oncology, but they offer better outcomes, different patient populations, and gyn onc- the opportunity to do some wonderful pelvic/bowel surgery.
As for Hem/Onc proper, if you are in private practice, I believe you tend to do mostly oncology. If you're in academia, you are more likely to do hematology and may even focus on it. There are 2 year medical oncology fellowships. I'm not aware if there are hematology fellowships alone.
Good luck.