Both are true, and both are false, depending on what individual AK are being discussed, when it was made, from what kind of steel and by who.
Here is a basic overview:
In many cases the pressed AKs really can be stronger than many milled ones but it is not the process of manufacture that is the issue. It's the steel used and the heat-treatments.
The very first ones made in Russia were NOT milled but stamped.
However they were not done from the correct type of steel and they had cracks developing. So the Russians went to a mild steel and milled the receivers.
They were good but had a tendency to batter over time and were not as resistant to denting as they wanted.
So next they went to a high grade of steel that was properly heat treated. These were the guns of legend, but were costly to make.
Next they went to a spring steel stock and pressed the receivers from sheets. These were also done correctly and were the full equal of the later milled guns, and in fact better then the earliest milled guns. These are the AKMs we see mostly today.
BUT... some other nations used the AKM pattern to make their own AKs and some of them went through problems with their first attempts too, because of wrong heat treatments for the alloys they were using.
And in the USA today with so many people making AKs from sheet steel some are not up to standards because people don't understand the process that must be done and how to do it, or some people understand but lack the proper equipment to do it right.
So is stamped better?
Yes, ------ unless its not.