Well, I went through the same decision some time ago. I just wanted a hammer, so I got the 638 and it is my mainstay carry piece in the summer. In winter I go for the bigger gunz and back to the 1911 or XD-40.
Someone wrote about the 638 being a pocket lint trap and that change in your pocket could jam the hammer. Well, anything in your pocket is going to get the lint from time to time. I inspect mine every day and I havent had a problem with any lint really. And, just how much lint would it take to make it malfunction? As far as the change... well I use a Galco Pocket holster.. or a Melt Sparks pocket holster. I carry my change in the opposite pocket, but on the occassion that I have had change in my gun pocket, it has never gotten into a position to cause a stoppage of the 638. Right now I am trying to wedge a dime or penny into it... its not working. When you pull the trigger the hammer easily kicks out the change. The only way I can get the change to stay in the slot is to keep the muzzle pointed at the ground anyway. It can jam a dime if it is in SA with the hammer drawn back, but who in their right mind is going to keep the hammer drawn in their pocket? My trigger pull is so easy in SA that its scary. If you dont believe me.. go to a gun store and try to get the change caught in it where it would stop the DA pull. As far as how they shoot. I have shot my 638 in IDPA BUG matches and it really performed well. In fact it did better than I had expected it to do. I scored better than some carrying small Glocks as their BUG.
Ultimately it comes down to what you feel comfortable with. I wanted the hammer, you may not. If you want a hammer, I personally think the shrouded hammer of the 638 is the only way to go for a small CCW revolver or a CCW pocket gun. You can keep your hammer and its not going to get caught on clothing during the draw or when firing from inside a pocket etc... I dont own a hammerless gun, but I suspect with proper care they too would function just fine.
Another thing to consider is the finish that you want on the gun. since its going to be concealed and your body tends to sweat, I would keep maintenance and corrosion control in mind when choosing the finish of the gun.
I hope my observations have been helpful to ya..
my two cents..