Speaking from the engineering side of myself, coil springs are MUCH less likely to break, and MUCH more resistant to weakening over time. For one thing, a coil spring spreads the load over the entire length of the spring, while a flat spring (like in an early revolver design) concentrates the force mostly at the anchor point, or at at the "V" , if of that shape. Given a choice, I would never choose a gun with flat springs, of any type.
From a historical point of view, use what was originally in the gun; as if one had a choice, in that case.
And that folks, is the correct answer. With a coil spring, each coil only flexes a fraction of the total amount. So the stress imparted to any particular section of the spring is very low. With a flat type spring, the entire spring flexes, and the flex is usually concentrated in one spot.
Here is a photo of two broken springs from my Colts. The split trigger/bolt spring at the top of the photo is probably the most common spring to break in a Colt type action. Notice where the spring broke, right at the base of one of the legs. This is absolutely typical of a broken trigger/bolt spring, this is where the spring flexes the most.
There is something else at play here. It is called Stress Risers. The comment about grinding marks creating fractures is spot on. Such marks cause what is known as a Stress Riser. A Stress Riser is any feature that allows a micro fracture to start. Poorly smoothed over horizontal grinding marks are a perfect example of Stress Risers. What happens is, as the spring flexes over and over again, tiny micro fractures start at the bottom of the 'v' left behind by the grinding marks. Here are a couple of typical Uberti mainsprings for their replica of the 1873 rifle. Notice the grinding marks left on the face of the spring. Those make perfect stress risers, and yes, I once had a spring just like this snap in half in one of my rifles. Right on one of the grinding marks.Frankly, I'm surprised Uberti rifle springs don't break more often than they do, given the extremely poor finish they have when they leave the factory. When I tune an Uberti mainspring, I always grind along the length of the spring, never across the width. A spring that flexes along its length is not affected by grinding marks also going along the length, only across the direction of flex. And I always finish up with very fine sandpaper to smooth the surface to a mirror finish so no irregularities will be left for micro fractures to start.
Going back to the broken Colt spring, notice the break starts right at the tight radius between the two legs. Another perfect Stress Riser. The other broken part in this photo is a bolt. Not a very common failure, but it does happen. With a Colt style bolt, one leg flexes every time the hammer falls, as the leg rubs over the cam on the hammer. This one gave way right at the weakest point, right at the hole for the screw it pivots on.
Why don't the hammer springs in a S&W or a Colt snap? Because S&W and Colt do a good job of finishing the springs and not giving micro fractures a place to start.
Yes, Ruger coil springs are indeed more durable than the old style flat springs in the traditional Colt type lockwork. I ain't saying they are going to break right away, but they are more likely to break. I love shooting my Colts, but a pair of Rugers always comes along as back ups. In the last ten years of Cowboy Action Shooting, in addition to the broken rifle spring, I have had to use one of my Ruger backups twice, and you are looking at the reason for each instance.