Both are good guns. The GP100 does look and feel a little "beefier" that the L frame, but I'm not convinced that the GP is any stronger.
It's true that the S&W has a nicer action, but a Ruger action is easily smoothed out with some 2,000 grit auto-body sandpaper.
Many will say that you shouldn't have to smooth out the trigger on a brand new gun.
Well, you could argue, "Why does S&W Performance Center offer a revolver action job for $165? Why doesn't
every S&W revolver leave the factory with the professional action job already done? Why should I have to add $165 to the already expensive price of a new L frame?"
^^^^^^^^
I don't really feel that way - I'm just making a point. To some people, it's worth a couple hundred bucks to them to buy the less expensive gun and spend a half hour tuning the action themselves.
I have owned both guns - a 686 no-dash, and a GP100, circa 1994.
I still own a 586 no-dash.
For beauty alone, I say the L frame wins, hands down. For rugged dependability and function, tie score.
Price, Ruger wins.
I would also claim a tie score for fit and finish - the GP locked up and fit together just as tight as any S&W I've seen. The blueing was as nice as S&W's modern blue.
I also had a Ruger SP101 - same story as the GP100 - a solid, reliable gun.
Of course, the GP100 was sold to fund my 586 restoration, and I traded the SP101 for an S&W Model 19-3.
With that in mind, I guess I lean more heavily toward Smith & Wesson.
All that being said, you won't hate yourself if you buy the Ruger.
P.S.
I don't remember what thread I poached these pictures off, but here are a couple pictures that stand as a testament of the strength of the S&W design:
You can see the bullet stuck in the barrel:
An x-ray of the gun, with all 6 bullets stuck in the barrel:
Yeah, the rounds may have been under-powered, and the barrel is junk, but the barrel stayed intact and the gun didn't kaboom - even after 6 rounds stuck in it.