I have the P228 and the P239 in 9mm. I am more accurate with the P239 because I can get more hand around it, but the ergonomics of the slide release leave something to be desired. The P239's slide release is recessed and tight, making it a real pain to drop the slide that way. There's just not enough lever to get a thumb on. The P228, however, has a normal slide release that is easy to hit. Before you say that I should drop the slide by pulling back on it with my support hand, I feel that my support hand should be moving back around the firing hand after I load instead of reaching up and over when my firing thumb can do the job just fine. I have started practicing doing that with the P239 because of the ergonomics, but it still does not feel natural yet. The P239 is way easier for me to conceal in an IWB, but the P228 does just fine in a SmartCarry under a pair of baggy shorts. So if concealability is your biggest factor, go with the P239. Otherwise, go with whichever one fits you best.
I have one of the extended 10-round magazines for the P239. It doesn't make the grip feel any better, unlike the extended P245 mag.