Up until police officers started riding in cars, the 6" revolver was very common. Sight radius and velocity being the main reasons, as well as the fact that 6" barrels are "holdovers" from the era when rounds like .38 Special were loaded with Blackpowder and more barrel was optimal for slow burning BP.
The very first S&W M&P's had 6.5" barrels, and in the 1890's BP was still very commonly used along with "semi smokeless" powders.
4" barrels for duty guns became more popular because being seated in a car with a 6" barreled revolver was not ideal. But, a lot of LEO's packed 6" duty guns in swivel holsters as well as standard holsters well into the 1990's and later.
The 4" barrel was born more as a "go between" between a snub and the "full size" 6" revolver. It was made to be handier to carry in the holster, and to balance better.
I'm not sure what my point is LOL I'm just bored and I guess I'm trying to say that 6" will give you a little better performance, and more recoil control, but you may find the 4" more fun to shoot, easier to pack and overall a better choice.
Don't forget the fixed sight 4" GP100's, too, the half lug 4" guns are out there on GunBroker and provide excellent balance, are lighter and still have the legendary strength of the GP100.
A guy killed a HUGE aggressive Brown Bear with a .454 Alaskan, so it shows that even a short barrel doesn't make much of a difference if the round is powerful enough. There's pics on the internet, I would have had to change my shorts after that one, if I were that guy
