There's nothing wrong with fixed sights, especially if you mostly shoot loads that are "regulated" to the fixed sights---like 158 grain standard pressure .38 Specials. And there's something neat about a fixed sight M&P---a classic, no-frills revolver.
Personally, I prefer the adjustable sights. Partly that's because I can see them better. On a lot of the adjustable sight guns, the front sight is situated higher, and the notch in the rear sight tends to be deeper, which means I can see the sight faster, both in a hurry, and when I'm taking my time.
The other reason I prefer the adjustable sights is that I've concluded that one significant reason for shooting a revolver in the first place is to take advantage of the increased precision and accuracy that a revolver usually offers over a semi-auto. If I just want to hit reasonably close to a moderately large target, then a semi-auto or a fixed sight revolver is fine. But an adjustable sight revolver permits you to adjust things so you can consistently hit a very small target, regardless of the loads you use, the manner in which you grip the gun, or how your own eyes "see" the sights.