What is the differance between a SIG 228 and a 229 other than caliber?

The 228 was / is a short version of the historic 226 and was designed for the 9mm. The 229 has a more hefty slide (fatter, heavier, etc) designed for the 357 Sig round. Also the lower frame of the 229 is also slighter wider.

Quoted: The 228 was / is a short version of the historic 226 and was designed for the 9mm. The 229 has a more hefty slide (fatter, heavier, etc) designed for the 357 Sig round. Also the lower frame of the 229 is also slighter wider.

In addition to this, the 228 has a mandrel-bent sheet metal slide with a breech block pinned in place.  The 229's slide is machined from a solid piece of steel.  The 228's extractor is completely hidden inside the slide, while the 229's is exposed.  The newer 226's are manufactured with the solid slide, just like the 229, while the older 226's were made just like the 228.

I don't know why, but I really like the sheet metal slides a lot better.  They are a little bit lighter and slimmer.

And the P228 is used in the military as the M11.  And it is the sweetest 9mm on the planet

The biggest differance between P228 and P229 is 1.

I may be wrong, but isn't the 229's slide made of stainless steel?

Quoted: I may be wrong, but isn't the 229's slide made of stainless steel?

Yes.  Blackened stainless steel.  And it seems the blackening doesn't hold up very well.  Fades to a grey over time.