Hi! May I check if it will cost me less miles if I buy a roundtrip business class (to US) ticket now VERSUS buying 1 way now first, then look out for flying-back 1 way ticket later?

Reason for splitting up the purchase of to and fro tics is cos i need time to clock my miles. or actually round trip booking at one go will cost less miles AS compared to buy one-way then one-way separately? Thanks!