Another vote for a humpback if you feel you might ever want a hammer to cock. With a 638 or 649 you might choose never to cock the hammer, but if you get a 640 or 642 I guarantee the day will come when you'll miss not being able to take that long range shot single action. (I have had both styles for many years, and know the attraction of single action, being a long time target shooter.)
As for fully exposed ("normal") hammers, I usually carry in a belt holster when I'm out and about, and have had no problem with an exposed hammer snagging on the draw (the few times I've tried it) but ONLY IF my cover garment is a shirt/jacket I can sweep back behind the holster before the draw. When I wear a sweater and have to lift it up, I HAVE snagged a hammer a few times. And I would NEVER trust an exposed hammer not to snag being drawn from the pocket of any pair of pants that I own and wear.
Around the house I frequently wear sweat pants (winter) or nylon shorts (summer), both with elastic waistbands. A J frame lives in the right pocket. In those pants, the difference between an alloy and steel frame in the gun is the difference between OK and drooping pants. If I always wore pants with a belt I might be able to keep a steel gun in a pocket comfortably, but that's not what I need for my circumstances. YMMV, of course, but think about this aspect of pocket carry before you pick a gun.