Personally, I prefer the newer models. The older stamped slide models had better balance, and cosmetically they were a little more refined. The newer machined slide models are more corrosion resistant, have a more reliable external extractor, and the trigger is a little more smooth and consistant. However, I've noticed some cosmetic issues with the newer P226s and P220s. I didn't notice any function problems, just looks. I've made it a project of mine to look into Sig quality in the last few years and it still seems that Sig put a little more refinement into the P229 over the other models.
"You can rest assured that we do not cut corners in the production of any of our pistol models. We sell nearly double the number of model 229 .40 S&W pistols than we do any other pistol model in our line to large law enforcement agencies such as the Department of Homeland Security and now the US Coast Guard. Due to the demand for .40 caliber pistols in the last couple of decades from the law enforcement community, there has also been a greater number of agencies putting their pistols through extensive and abusive testing. All of our pistols are of top quality, but we do put a little more into refining the model 229 because of the market demand for it. If your product fails domestic agency testing, then you can kiss contracts goodbye. We won't have that."
-Kenneth Horne, production manager at Sig Sauer from "Law Enforcement Annual in 2007"