“I want to have free time to explore and delve into extracurricular interests. Basically (please don’t take this the wrong way) I DO NOT want to work too hard”
Do you have a third choice? MIT is probably not the right choice for folks who don’t want to work too hard.
“How important are departmental rankings for undergrads trying to go in the STEM fields?”
For computer science and Silicon Valley, not much. For Wall Street, I don’t really know (I went with Silicon Valley) but probably much more so. However, my guess is that Wall Street would be very happy with Yale regardless of whether any specific department is ranked below MIT’s equivalent department.