The inside diameter of 3/4" Pex is smaller than the inside diameter of 3/4" copper. I am a great proponent of recirculating systems that will provide instant hot water. The 1/2" pipe will not supply enough flow for a master bath. Compare the areas of 1/2" and 3/4" pipe and you will see it is a huge difference. If you have forgotten, Area = 3.14 x radius squared. Radius is 1/2 of the diameter. I don't know much about Pex other than the size difference.