I did a month at Monte and learned 2 things about the program. First the residents work harder than most programs- I met a resident who had just completed 2 months straight in the units. They all described the program as very intense.
They also have incredible fellowship placement, as good as any that I have seen. Many stay at Monte, but they send people all over the country in even the most competitive specialties. Can't speak for NEMC.
For Heme-Onc; I have heard that BU's Heme-Onc dept. is not in the greatest shape right now, but will likely rebound. I don't know how this would affect one's fellowship application though. They seem to have the best luck at placing people into good GI, Cards, and GenMed faculty positions. like buz said, Monte is a tough program, but has a strong rep that will take you anywhere you want to go. Don't know that much about it. Very popular choice among my classmates this year.
I haven't enjoyed Tufts-NEMC that much as a student, but their fellowship matches are excellent all-around, and their subspecialty medicine system (which includes time on BMT unit) might be a good fit for someone who knows they want to specialize. However, I know a few Heme-Onc fellows at NEMC who say that their clinical experience was excellent, but research wise, the dept. is mediocre. NEMC is also quite disconnected from the more prestigious hospitals in Boston, while BU seems to have more of a cooperative spirit.