The major difference is that the 228 is 9mm only. Also, the 228 has a formed metal slide, with a welded barrel bushing and a pinned in rear action, where the 229 has a machined steel one piece slide.
The design of the 229 was originally made to accomodate the added stress of the 40 S&W, and then rebarreled to 9mm, with 357 Sig as an added bonus.
There are hi caps available for both guns.
I carry the 229/40, as my personal preference, in 12 round hi caps, with 165 gn GoldenSabers.
For all intents and purposes, in 9mm, they're the same gun as far as shooting performance goes. The 229 was developed with similar frame dimensions as the 228, thus the similar performance feel.
Check out these links. The first is Sig's homepage, and the second is an informal SIg page with lots of good articles and reviews with compare/contrast between the models, as well as some history.
http://www.sigarms.com/
http://www.remtek.com/arms/sig/index.htm
Tom
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