My learning to fly was split between tandem and side-by-side; I always much preferred tandem when I was learning. My reasons were:
-Right hand circuits!
-Not having to look at my flying instructor, who scared me (at-least did when I was in the UAS, subsequent civil instructors were usually nicer).
-The sense of independence in the aeroplane, which I think helped my learning process, I tended to behave much less dependently upon the instructor.
Mostly (3) to be honest.
G