Best .357mag J-frame?
If it has to be an actual J-frame (S&W), I'd say the 3" M60. The 3" barrel gives more velocity and thus better overall bullet and hollowpoint performance. The 3" also gives a longer sight radius for better practical accuracy. Any J-frame that is heavy enough to make some use of .357mag manageable will be too heavy for pocket carry anyway, so the 3" won't be at much of a disadvantage v. the 2" for concealment as either will pretty much disappear IWB. However, the extra inch of barrel means almost 2oz more weight to make managing .357mag a tad easier (yet still light enough for easy CCW).
Still, I think any S&W (or Taurus) snub is too light and really not up to the task of managing .357mag. It is far from a pleasant experience. Truly the best "J-frame" (in this case, generically meaning "small framed snub") would be the Ruger SP101 (I do not have one BTW, I prefer my snubs in .38 in which case the S&W and Taurus sized small framed snubs are perfect). The runner up would be the 26oz, 6 shot, Rossi 461/462 if you get a good one (mine isn't fully reliable yet, the cylinder binds on .357mag but not on .38spl). With either, the extra weight helps with managing .357mag quite a bit, and the SP101 is quite a robust design. The SP101 also offers a 3" barrel for the same advantages mentioned for the 3" S&W 60.
Now, if you take .357mag out of the equation, and plan to shoot .38s, I'd say the best J-frames are the 442/642 (for pocket carry) and the 36 or 60 (preferably with a bobbed hammer and DAO conversion) for IWB. I will probably add a 3" M60 sometime in the future.