I think your choice of grip and holster is more important to the end result.
I shoot a 640 Pro Series. I think it is the best of the three, but it's not perfect. I find the sites lame. The sight radius is too short to make use of rear sights other than the standard J frame gutter. Pinned front sights let you choose what you want in front, and that's all you need. The sights on most SP101 and the K6s are fine.
The rear site on the 640 hooks on my pocket, and it works very poorly with pocket holsters. I have two different Galco pocket holsters, and I've tried other brands as well. I have other J frames to compare it to, and the 640 has problems drawing. Because of the shape of the rear sight, it has to be drawn by pulling the butt forward so the holster rotates away from the site instead of into its hook. Not all pocket holsters even have a tab to catch the pocket on the top of the holster. The Galco horsehide does, but it's pretty broad for some pockets. Just remember that the gun rotates out of a pocket holster. It does not pull straight out. Look at how it's going to rotate and see which combination will work for you.
The 640 is lighter than an SP101 but it's not really light enough for pocket or ankle carry. I use it for both, but the aluminum J frames are better for this. The Ruger with its additional weight and hammer spur is not really good for pocket or ankle. You might not care if you're sure it will be IWB/OWB carry, but will the weight of the Ruger make magnums easier to shoot? In my experience, the 640 is plenty heavy for full magnums (and the Kimber weighs the same). I can see how the 340PD might be too light with some grips, but I've shot it with wood grips and it's not as bad as some people make it out to be. People write or talk about the speed of follow up shots, but from my experience, it's just Internet conjecture. I have no problem getting fast double-taps on target with a 340PD and .357 magnums, and I'm not special. I've seen others dispell the follow-up speed myth with plate drills. The best recoil-absorbing grip I've found is the Crimson Trace CT350. It's not that the recoil absorption is so much better than any other (it's about the same as any soft rubber grip), but the quality is great. It's much nicer than the CT305 or CT405, and better than Hogue or Pachmayr. Even so, I prefer wood grips, but haven't found a good one to recommend.
With any of the all-steel guns mentioned and a good grip, .357 is not hard to shoot -- but you've got to build up to it. Start with lightweight .38's and make sure your fundamentals (grip, trigger control) are solid on this platform before you move up to greater recoil. That's easy to do with a 38/.357 revolver -- take advantage of it. People that shoot the 340 and quit aren't doing it right. They come with a grip technique that sucks, get jacked by a .357 and quit. It's not the gun.
I like the elegance of the minimal, forged frame on the 640 and the five-shot cylinder with the bolt notches in the web of the cylinders instead of the outside edge of the chamber. The trigger is smooth (after break-in), and the gun is clean in appearance. I've considered using the moon-clip feature for the loaded rounds to make ejection more positive, but I carry extra rounds in a speed strip and not in a bulky moon clip or speedloader. The Kimber has nice touches like recessed chambers. I've read others rave about the trigger, but I didn't think it was any better than the S&W. It's definitely a nice gun. I've complained before about the excess logos on it though.
I've never seriously considered the Ruger because of the hammer spur and excess weight for my primary carry methods (pocket and ankle), but it seems like a fine gun and Ruger might be a better company than S&W. I can tell you the quality control on several new S&W's I have is poor. I've sent more than one back to fix problems. Barrel to cylinder gaps are big, cylinder lockup is poor, and there are other problems. I wouldn't buy even a new one without checking it out carefully. There are guides on how to check out a revolver. Do it before you complete the transfer.