On that 649, is the "shrouded" hammer easy to cock? As in a SD situation would it be an easy one handed draw, cock and fire single fluid motion?
Short answer is that no, it's not "easy" to cock in the manner you describe. It takes deliberate effort to do it smoothly.
But as previously mentioned,
your first shot should be DA for speed. Cocking the hammer manually will only slow down your time from draw to first shot.
One way to shroud the hammer and still have it fairly easy to cock is to buy a 6-shot K-Frame (as mentioned above) and add a set of the old Bianchi Lightning grips (as pictured below). These seem to fit the older pre-lock K-frames better than the new ones. And they're actually comfortable.
S&W Model 19, .357 Magnum, 2.5" Barrel with Bianchi Lightning grips
If you can't find these and want a pair, I have a couple spare sets I can sell reasonably priced.
I'll second Sgt127's suggestion of a 3" Model 65. Medium K-frame, round butt for concealability, .357 Magnum, fixed sights, heavy bull barrel for balance. Everything you need with no frills. The Model 64 is similar, just in .38 Special.
Re: Calibers - if you polled people, I'd say that most who carry a 5-shot J-Frame .357 probably carry it loaded with a quality .38 Special +P load. Several reasons for this - less recoil than the magnum means faster follow-up shots; in urban areas, the .357 may be more of a liability if you miss; less noise which can be disorienting; less muzzle flash, especially from a snubby.