Headsets

First post folks so please excuse the ignorance.After many years cycling I,ve bitten the bullet and set myself a project to build my own bike based on a px rt58 bare frame. I,m informed I need to fit a FSA -C-40 Acb tapered headset which I,ve located on the web but it gives two options ie 9mm or 15 mm cone spacer.Does it really matter which at this stage as when or if I cut the fork head I can fit various spacers between stem.I appreciate that this is an expensive way to build a bike and could by a complete bike cheaper than the sum of it,s parts but it will be a project to keep an old man busy. Thanks in advance for any help :/