I want to offer a different opinion than Gutonc on this issue of Heme vs Onc vs Heme-Onc boards. This is based on the opinions of several attendings/PIs in my fellowship program (a very academic program) and on my own perceptions.
If you are going into private practice, then double boarding is almost essential.
If you are going into academics, then single boarding is fine. In this setting, single boarding in Heme is much less useful than single boarding in Onc, even for people interested in hematologic malignancies. At the institutions where we rotate as heme-onc fellows, everyone who actively does transplant has boarded either in Heme-Onc or in Onc, although the attendings at my program have said that at smaller academic centers, the rare folks who board only in Heme may sometimes do transplant. My impression is this is largely because, as someone else said earlier, Heme boards really benefit those who want to learn non-malignant hematology. If you are interested in transplant, the advice from attendings/PIs in my program would be to double board, but if you were insistent on single boarding, we would say you should do Onc but also do some transplant rotations as would be required for Heme boards.
As for my own experience, my academic and clinical interests are mostly in malignant hematology, and I went through this exact same set of choices at the end of my first year of fellowship as I had hoped to shorten my length of training. In the end I decided to do both Heme and Onc and am very happy with my decision. Feel free to PM me if any of you have add'l questions.