Posture #2, holding the tan tien, is more of an isolation exercise. The first posture has everything. It includes the movement of all of the organs. If you got the movement of each organ in posture #1, you probably don’t need to spend time with this one unless you specifically want to work on and strengthen the flow out the back that corresponds to the shoulders’ nest (and its matched flow in the legs).
In posture #2, you are emphasizing the lower tan tien, but as soon as you do that you are better off working from the CSF and the brain ventricles. I’ll also comment that I like posture #7 much more for waking the lower tan tien. That said, I’d argue the the whole business of bai hui and the peak of the yang meridians is better understood from the perspective of circulating the CSF from the 4th ventricle. Because that’s really the mechanism that is responsible for the felt pressures that Bruce describes in module 4 video lesson 10. Another comment, the balance seems way more complete in posture #1 as well.