I have much better memories of "L.A. Confidential", but I'm sure it's considered sacrilege to rate it above "Chinatown", which I haven't seen in a very long time, so I can't really make an informed decision.
I'm not gonna lie, if you have a short attention span, you're gonna like LA Confidential more. It's faster paced and more action packed. Chinatown is slow paced, yet brilliantly made and Nicholson's best overall performance.