Sig-Sauer P229 question about magazine interchangability
 

Got a P229 with .40 S&W and Sig .357 barrels. The magazines say .40/.357 on them. I was fooling around this morning and noticed that the 226/228 9 mm magazines appear identical. I loaded up some .40 ammo and don't see any difference in the spacing internally. I know this is also marketed as a 9 mm, but don't know what mags come with it in that configuration, or whether it's just a simple barrel change like it is from .40 to .357 Sig. Will my 9 mm 226/228 magazines work in the 229? Thanks.

As an aside, why would anybody want a big, heavy 9mm in the 229, when you could just have a 228? I don't quite get the reasoning behind this, other than marketing.