Just brought our 4.5 year old DD, and we did both. We started with BTMR for nostalgic reasons - it was going to be her first roller coaster, and both DH and I have memories of riding BTMR as our first roller coaster at around that age. She loved it and immediately wanted to go again. I knew she would, as she is my little daredevil. 7DMT is much smoother and less intense, as others have said, so I would recommend starting with that one if you aren't sure how your son will react.
DD liked 7DMT even better than BTMR, I think because of the cute little scenes. This 30-something mom who hadn't been on roller coasters in 15+ years preferred the 7DMT too