I can offer extra reassurance on GRR - our son was just 42 inches when we went and GRR felt very safe. The belt fit him well, it is not a wild ride, and you can almost always arrange to have him sitting in a middle seat. In fact, it was one of his favorite rides and at one point, we went on it 6 times in a row.
Now, I did not feel as safe with him on BTMR - I felt like he was going to slip under the bar at anytime and the height requirement is only 40 inches for that one. Now, I am not discouraging you from BTMR - my son did not slip under the bar and I am sure it is perfectly safe. But my perception of safety was influenced by alot of things, not just height requirement. And if I spent the whole ride worried about him, then I didn't have much fun either. You can search on youtube for videos of almost every ride at DLR and that helped me to know what to expect. (It also helped my son).