The SP101 is heavier and tougher.
The Ruger can also be fully field-stripped by a non-gunsmith, and springs easily changed. You can shoot a huge number of hot rounds through it without hurting the gun; used SP101s are virtually always in good shape (barring really stupid handloads) because you can't physically shoot enough full-house loads to hurt the gun before your wrists give out!
Rugers have no sideplates; the action and grip frame "forks up" into the bottom of the fully solid frame. Rugers also "lock up" at both the crane and rear of the cylinder. Swing the cylinder out and push on the stud at the center of the ejector star (rear of the cylinder) while looking at the crane - you'll see the "interlink" and the locking latch that holds the crane firm in the frame.
Upshot: the Ruger design is worlds past the late-19th-century S&W design. But there's a price: a bit of extra weight, and the trigger could usually use a bit of tuning.
Now, the S&W isn't a bad gun. Especially if you shoot mostly 38/38+P and save 357s for carry and enough practice to know where they print. It's lighter for carry.
In my opinion, neither gun is small enough for front-pocket carry. SOME would say that the S&W is so compatible. That's really what it comes down to, because if you agree that front-pocket carry is out of the question for both, then you've got to do an IWB or other belt rig, or a fanny pack, or a shoulder rig. In those cases, the extra five or six ounces on the Ruger won't matter much...except of course to help tame recoil some!
Now, all that said, both guns will work. And since both Ruger and S&W are capable of every once in a while dropping a lemon, RUN THE CHECKOUT:
http://www.thehighroad.org/showthread.php?s=&threadid=1430
My preference is for the SP101. BUT an S&W J-Frame 357 that passes the checkout well will always be picked over a "loose and sloppy" Ruger.