I have the four inch and for my purposes I wouldn't trade it for a 6", but then I own other .357s with 6" and longer barrels. One of the primary purposes for my GP100 is for open carry while hunting or exploring the back country. IMO, the 6" is too long for actual carry. The big thing is that when you try to sit down to drive, the barrel either pokes into the seat and pulls your belt up around your neck or the whole gun twists causing your belt to cut you in two.
If I had no intention of ever carrying it, or carrying it in a shoulder rig, I would go with the 6". My tests have shown that the .357 really picks up speed fast as your barrel length increases. So, obviously, by choosing the longer barrel you are going to get much more performance out of your ammo along with the longer sight radius which will make the gun easier to shoot accurately.