Well, the difference between a typically non-procedural IM specialty like Heme-Onc (although they do do BM biopsies and a few other procedures) and general surgery are vast.
I'm not sure where you are in the training route, but if you haven't done your clinical rotations, this should help you immensely. Generally students either find they love or hate the OR and the surgical lifestyle. When this occurs, it generally makes the decision somewhat easier.
But let's say you've already done your cores and still can't decide. What are the differences between the two worlds?
First, exclude the issue of "patient contact" from the equation. Despite the propaganda, surgeons have plenty of patient contact - patients are seen pre-op, intra-op and post-op - often for months or years at a time. I can honestly say I spend as much time with patients on surgical services as I did on medical services....not including rounds (because I don't count standing around in a large group in the patient's room as valuable patient contact time). So lets assume patient contact is equal or at the very least comparable betwen the two specialties.
While gen surg and its subspecialties obviously deal with sick patients, I would venture that more Heme-Onc patients are critically ill and more often. How often do you want to deal with the issues of critical illness, death and dying - especially in the young (children and adults)?
How long do you like to wait to see the results of your interventions? With Heme-Onc its longer and you run the risk of recurrence - obviously this is also the case with some surgical patients as well, but again I would venture its more common with the Heme-Onc patients.
Gen Surg tends to have more in-house call and more emergencies; most Heme-Onc patients, at least initially, are consults and while there are emergent crises, less frequent.
At any rate, I'm not really qualified to speak in detail about the daily activities/lives of Heme-Onc physicians. I do know it to be one of the more competitive IM specialties, but perhaps others can give you a better picture of what the field is like than I.