Research in cancer or cardiovascular?!


I am a fresh img graduate, I have done 2 Hands on electives last year during my medical school in cardiothoracic surgery and general surgery… and i got 2 LOR.
Unfortunately i failed on CS. But planning to take it after my step 1 exam.
Recently i had opportunity to have 1 year observership with research in cancer institute in USA and in the same period a opportunity to be a Research fellow in Harvard with cardiovascular doctor their. So i have to choose.
So my question what is better for me in the future if i want to have IM residency?? And is it true my chance to have General surgery is very little after failed attempt in cs ??