What do you want it for? Have you held it?
If both were available off the shelf today I'd probably buy the 6", but I have a 4" .22lr revolver already [Ruger SP101 8shot].
If I didn't have a .22lr revolver, I'd probably lean toward a 6", as it wouldn't be a 'carry' gun for me. If I am going to carry in my state, it will be a larger caliber than .22lr.
HOWEVER, I would hold them at the LGS and hold them in the sight-acquisition/firing position for as long as they would let me to mimic what my arms will feel like after 5 cylinders are pushed through it. I like the idea of the 6", but if my hands shake with the weight too much, the 4" would be more accurate in my hands.
It all depends: on your health and what your purpose is.
Just FWIW, I am selling my 6" GP100 and keeping the 4" version. Of course, if I want more power, I have a 7.5" Ruger SuperRedHawk in .44mag, so I didn't need the extra power the 2" more on the .357mag will give me.