P228 has a stamped & folded carbon steel slide, with a pinned breech block. Also has full-height slide serrations. The P229 has a milled stainless steel slide with half-height slide serrations. P228 has an internal extractor and the P229 has an external extractor. The P229 is about 1.5oz heavier due to the stainless slide.
While either pistol will serve you well, many folks report that the P228 balances better. I've used a P228 as my carry gun for a number of years now. It shoots well and I've only had one issue with it (broken trigger reset spring -- easy fix).