Joists and rafters

A roof carpenter has recomended to me to place my rafters at 450mm centres instead of the usual 600mm. In his opion there is less spring,straighter ceilings and insulating batts fit more conveniently for not much more expense. Now another carpenter has said to place the rafters at 600mm and have the additional ceiling joists at 450mm. I dont see the point of this as you would have rafter ast 600mm then joist at 450mm then 150mm away the next rafter.?? I am now confused and reached the limit...

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