I too have both. I find the P239 much easier to shoot well with.
My P239 is chambered in 357SIG, which basically puts it in the 357MAG power level, so its also a bit more powerful than the +P .38's the 642 carries. It also carries 2-3 more rounds, depending how you load the gun.
I dont have CT''s, as I'm not a laser fan, but I do have night sights on the P239, which work well.
My only real complaint with the P239, is the stock grips. I find them a tad skinny, and would prefer more swell through the palm. I replaced the factory grips with Houge wood grips. It would be nice if SIG would offer something a little fuller in their factory plastic grips though, as I prefer them to most of the others.
Shooting wise, I think you'll find the P239 is the better gun, pretty much all around. Its much more comfortable to shoot over the J frames, and you can shoot it all day long with no discomfort. Shooting the J frames, and especially the Airweights, can quickly become painful, especially if you practice with full power loads. A box of 50 is usually all my hand can stand at an outing, and its usually sore the next day to boot.
The triggers on both are very good, and the P239 can be had in a couple different configurations if you wish.
My groups at 15 yards with the P239 are usually about the same size as my groups at 7 yards with my 642's or 60.
If your considering the P239, I'd also look at the P229 or P250. They are basically the same size as the P239, but carry about half again as much more ammo on board. They all shoot and handle the same. The P250 is also DAO, and has a VERY nice trigger.