pH plus is sodium carbonate (sometimes called sodium hydrogen carbonate) . It is exactly the same thing as Arm and Hammer Super Washing Soda (also called Soda Ash or Sal Soda) found in the laundry aisle of the grocery for a LOT LESS MONEY than the pH Plus (and probably sitting right next to the 20 Mule Team borax). It is not the same as baking soda (sodum bicarbonate) that your pool store sells as Total Alkalinity increaser for a lot more money that the grocery store does. Pound for pound it takes about twice as much borax as soda ash to raise the pH the same amount. HOWEVER SODA ASH WILL ALSO CAUSE A VERY FAST RISE IN TOTAL ALKALINITY WHICH IS OFTEN NOT DESIRABLE so borax is preferential since it has a minimal impact on TA.
It si really a comparison of apples and oranges since the two chemicals have very different effects on your pool's water balance.