I think it is easier to get a really good flat spin first. It is easier to back a horse into a rollback at any time. Once a horse is backed into rollbacks, it is a lot harder for most to spin flat without stalling out, backing up or hopping.
Plus, on the roll back, even if a horse has been spinning really good, it is very natural to back into the turn to follow the cow. The cow will
'draw' the horse through the turn. You have nothing to help you spin, so I think it is a lot easier to keep a correct spin if you teach it first.
I have taken a lot of reiners and put them on cattle a lot easier than I have taken cutting flunk-outs and put a really good spin on.