a stamped slide is machined from basically a flat peice of sheet steel, then folded into the desired shape. The internals are then pinned in with roll pins.
This is both inexpensive and lightwieght. the disadvantages are the same.
a machined slide starts a chunk of ordnance grade gunsteel, and is then cut into the needed shape. This both stronger and heavier... and more expensive.
I own 9 SiG Sauer pistols, ranging from the mid 1980's ( a p225) to one just a year old ( the black water 226). 5 of my sigs have stamped slides, the other 4 have machined slides.
The machined parts are no doubt strong, but no SiG ever had a rep for being cheaply made, or undurable.
IMHO, the difference is purely new vs old tech. Now that CNC machining is the norm, there's no reason to make a stamped steel anything.