Because of it's size (driven by the idea that 9mm is underpowered), the 3" seems to be more desirable (6486 3" round butts were made compared to 3784 4" square butts) but there were more 3" models produced than the 4" model. Either one is an excellent revolver, and getting your hands on either one is bonus. I've got a 4" square butt, but would love to get my hands on 3" round butt!
I wish S&W would put them back into production, as well as many other members of the forum. I mention the power issue, because I think it's fair to compare it to the .38 spl, not the .357 Magnums. Here's a little photo comparison I did; the speedloaders for the .38/.357 are for 7 round L Frames, the 9mm is for the 547. Notice the size/power ratio, particluarly for the .38+P to the 9mm. That's not even 9MM +P, I might add. 9mm also performs better out of short barrels, but I prefer 4". I think 9mm is well suited for revolvers, after all, S&W has been chambering .45 ACP for revolvers since 1917 and still does. Even moon clipped versions are a good choice, so much so that I'm considering coverting a Model 64 or even a Model 10 to 9MM. However, the 547 is magical since it doesn't require moon clips due to that special little pin and berylium clip system.