Regarding SIPS.. We typically run common rafters on 4' centers and use curtainwall panels running up and down the roof. There is minimal waste and very little finishing work. The problem with running panels horizontally is you have a lot of seams to clean up in a vaulted area.
And for the customer that wants a wood ceiling, a chamfer along the top edges of the rafters makes a nice little pocket to drop in 1x6's in horizontally and then apply the panels over them in any direction you want.
