Get the 642. I'm planning on one myself at the Fort Worth gun show this weekend. Still trying to decide on the laser grips or not. At Sportsman's Warehouse the 642 is $339, and $499 with the CT 305 laser grips. I'm leaning towards the laser since this gun does NOT have a pinned front sight, making sight change more expensive/difficult, and if you believe the stats, it is more likely to be used in low light.
I got the 640 (stainless, 2 1/8 barrel, 23 ounces) a couple of months back because I didn't like shooting the 340. Never considered the 642 due to lack of pinned front sight, but have reconsidered since the 640 is just a hair too noticeable in my pocket.
Good luck!