Actually, this is pretty easy...
...seriatim:
The P225 and P228 have never been offered with other than stamped-steel carbon slides with pinned breech blocks.
The P229 has never been offered with other than a milled stainless slide; it was, of course, designed for the higher-pressure .40Auto cartridge and has since been chambered in .357SIG [what a great round!]. It still is offered in 9mmP, too.
The P226 was originally an upscaled P225 with double-stack magazine capacity, thus was available only with a stamped-steel slide. Now that it is offered in the "hotter" calibers of .40Auto and .357SIG, it comes with the milled-stainless slide.
But wait...there's more.
The original P220 with stamped-steel slide and alloy frame is now being offered in an all-stainless version [no, not the Sport model, but the P220ST].
Likewise, the P226 sometime later this year.
All that said, you can't find a finer, out-of-the-box combat pistol than any of the SIG Classic series...IMO, of course.