PW is a T4 for unit. It has a higher tech cost.
Katy hits the field easier and earlier.
In addition Katy usually has "juicer" targets to hit due to the reliance of Ostheer to support weapons and the OKW trucks.
That makes Katy usually have a bigger impact in game then PW.
PW and Katy are both T4 units and are you really saying to use Katy vs Truck? Soviet and British are also relying on support weapons...
Then I don't know how you can come to the conclusion that Katy have bigger impact than PW if you don't build PW. You need to compare both when they are used.