Mach 5 at 28km and mach 6 at 70km are not all that different. Mach slows down with altitude. And specific altitude numbers dont matter much if you are aiming for orbit. All that really matters is horizontal velocity. The first stages of both schemes are relatively slow. Both get the bulk of thier orbital velocity from thier second stages.

SpaceX takes a very non-aggressive approach with its first stages. When they detach they arent moving nealy as fast as other rocket first stages. That is done so they dont slam back into the lower atmosphere but it means relying upon second/third stages to do the horizontal work. By the raw physics, it isnt the most energy efficient approach. It is a compromise to facilitate recovery.