"It depends off course," - But we need to stay on course.
'IF' the Surface Book had the dedicated gpu then I'd go for the Surface Book i5. If, however, it doesn't then the i7 on the sp4 will afford you better graphical performance over the i5 on the SB. Not a great deal but it's there.
The i5 vs i7... not that much in it to be honest. Sure, yes, there is a difference but there's other things at play here. Slightly bigger screen (SB) compared to the Surface pro 4. Will you be okay with the Surface Pro on your lap? (I am)