Not to disparage the Ruger revolvers. I think they are excellent. In fact, my dad left his Ruger Speed-Six in a drawer for over ten years, and it shot fine when we pulled it out. However, when you compare the J-frame Smiths with the Ruger sp-101, the Smiths are smaller and lighter, and therefore easier to carry.
All the SP101's are about the same size and weight (.38. 357, 9mm, etc.). The Smiths come in various styles (including stainless, airweight, and airlite). To compare apples with apples as closely as possible, put a .357 SP101 against a .357 model 60. Both shoot well and will be extremely reliable. The place where the Smith wins is that it is smaller and lighter than the Ruger (in fact, I have a .357 model 60 around my left ankle as I type this). And when you compare an Airweight or Airlite to a .38 Ruger, the difference becomes even greater.