As Gordon said you really won't know what the velocity difference will be, it could be a lot, it could be very little depending on the loads and specific guns.
For example with the original 357 Magnum load my 8 3/8" Smith only picks up 30 fps over my 4" GP100 (1,416 fps vs 1,386 fps). You'd expect there to be more of a differnece with that much barrel and a slow burning powder, but with these particular guns that's all there is.
I would definately go with the 4" gun, or even better a 3", instead of the 6" for CCW.