The Pro Pink has a higher R factor and higher price. The Styro is cheap, has a lower R factor and since it comes in thinner sizes, it is used where the thinner insulation is all that will work, like the door.
The door is actually a combination of pro pink and “great stuff” spray foam. You want EVERY air pocket sealed in the door.
If you can find thinner Prop Pink (I have only been able to source 1.5″ thick.) then by all means, do the substitution.
That’s funny, I looked all over and could not find Pro Pink in the 1.5″ size at all. All I could get was 1/2″, 1″ and 2″ at my local Home Depot, so I bought one sheet of each. I don’t plan on using any styrofoam.
I don’t have my door built yet, but with a chunk of 2″ insulation blocking the door opening my keg box will get down to 25 degrees (using an Edgestar freezer/fridge) on the freezer setting.