Cliffnotes. Girling boosters are flimsy.
A while back when I was trying to understand why the e30 brake pedal wasn’t as stiff as a Porsche pedal, I figured out that my ATE booster was flexing. A couple guys encouraged me to get a Girling booster because they were apparently made of stouter stuff.
It wasn’t. I put the Girling booster on and the behavior was unchanged. In fact another problem seemed to pop up, where the booster wasn’t holding vacuum which was impacting my attempts to heel N toe. It took a while for me to decide that the problem wasn’t my imagination and and get around to doing something about it.
Swapping out the checkvalve last month didn’t help so tonight I swapped out boosters. Once I had the Girling booster out I noticed how light it was. So I weighed both boosters and I found that the ATE booster is about 1/3rd heavier, ~5.5lbs vs. ~7.2lbs.
AFAIK there is no functional difference between the two, but given the issue of booster flex making the pedal soft, stouter is might be better.