Milled is stronger because it was designed to support .40 and .357 later on in the late 80'; the p226 was originally designed in the 70's for 9mm hence the frame and slide were different at the time and manufactured with the stamped techniques that Germans are known for.
The P226 has seen different upgrades throughout the years, one was the frame being a few micro-milimeters wider around the slide rails and dustcover in order to solve the problem of cracked frames seen in the 80's; after that the design suffered more changes when .40 and 357 were introduced in the platform, the frame suffered more changes again with the most obvious being the slide milled instead of stamped, in order to support stronger recoil forces. Stamped slide is noticeable lighter that a milled one, the stamped slide used a different ejector and the fire pin block/assembly is held in place by one rolled pin (that used to be crushed over time and needed to be replaced frequently in the german design, most likely when hot 9mm loads were used), this does not happen in the milled version due to being one solid piece throughout.
The P226 has seen different upgrades throughout the years, one was the frame being a few micro-milimeters wider around the slide rails and dustcover in order to solve the problem of cracked frames seen in the 80's; after that the design suffered more changes when .40 and 357 were introduced in the platform, the frame suffered more changes again with the most obvious being the slide milled instead of stamped, in order to support stronger recoil forces. Stamped slide is noticeable lighter that a milled one, the stamped slide used a different ejector and the fire pin block/assembly is held in place by one rolled pin (that used to be crushed over time and needed to be replaced frequently in the german design, most likely when hot 9mm loads were used), this does not happen in the milled version due to being one solid piece throughout.