Phalanx Pikemen vs Silver Shield Pikemen.
The only real difference between them is the improved morale and stamina of the Silver Shields. Is that worth the extra cost and a two turn build? If Legionaries are available then certainly No. A Phalanx pikeman front line supported by some units of Legionaries gains as the Legionaries are just better at fighting.
So, building progresses on Legionaries where possible. Silver Shield pikemen will be used where no other alternative is available and then only a few units scattered about to anchor a battle line. Besides the Legionaries are cheaper.