I've been shooting both for decades. Fine guns, no doubt, but I prefer the looks of the M15. S&W sight blades are slightly fragile and a blow at the right point can bend or damage them, though I've only seen this once. If you get used to the rear sight picture on the Model 10, I don't think it's any better or worse than a Model 15s sights.

I believe the Model 10's heavy barrel makes it a bit easier to shoot well. The 15 always seemed a bit light up front to me, but it will certainly shoot as well as the Model 10. The Model 10 will usually shoot heavy bullets to point of aim or close to it.

My personal observations only and I don't expect everyone to agree...