Since FM weighed in, I will.
Three years ago I did a head to head.
After which, I bought the Ruger & returned the Smith.
The Smith shot fractionally tighter at 25 yards off the bench, but the Ruger had cleaner machining in the barrel.
Both triggers can be worked on.
I don't much like the sights on either.
I dislike the internals & lock on the new Smiths.
I'm not happy with the MIM trigger on the GP, but MIMs are minimal in the GP.
The new Smith uses an electro-chemical rifling that doesn't always work well with lead.
The Ruger still uses conventional rifling methods.
I trust the Ruger to be more durable & to be less inclined go out of time over the long haul.
The Smith can be tuned to a "better" trigger, but not by much.
I find the Smith easier to get the guts out of, down to the individual part level.
The Ruger's modular trigger group is not as simple to break down.
Both brands have been subject to Canted Barrel Syndrome in recent years.
Both brands tend to need grip replacements.
My latest make-over revolver project was a vintage 686 4-incher, which (among other things) got Bowen sights and custom grips.
My current not-yet-finished make-over revolver project is a 7-shot Ruger GP with 2.5 barrel in .357, which has Bowen sights so far. More work scheduled.
I own a handful of GPs, and my oldest is 30.
I had an L-Frame Smith 35 years ago, when they first came out.
I've had a couple since.
I think the edge for durability is clearly Ruger, the edge for refinement clearly S&W.
Both can be upgraded, accuracy about the same on both.
The only two issues I'm aware of with Ruger's sights are a tendency for the rear sight pin to walk (easily fixed), and the way the rear sight's designed.
In some holsters (and rare cases) that bear against the sight enough to depress it, the elevation screw can work loose under continual wearing, changing point of impact.
I've never experienced that, and never known anybody who has.
I've started to go with Bowen's sights on revolver upgrades in both brands, mostly for visibility.
The white-outline rear blade can be a little dim in both.
Staying factory, S&W probably has a better rear sight
When my son-in-law recently decided he wanted a .357, I steered him into the Ruger, now at the gunsmith getting a work-over.
Of the two brands in CURRENT manufacture, I think the Ruger is the best choice for longevity & quality overall.
Both need after-factory work to maximize beyond factory levels.
Either one will be adequate out of the box for most shooters, aside from the QC issues common to both.
Denis