Hi,
I have a T/C 10" barrel, a Super RedHawk 7,5", a SW 29 4" and have just exchanged the 6" barrel of my 629 for a 3" one.
Like it's been said, it all depends on what you want from the gun.
My new 3" 629 shoots great, accurate enough to put down the chicken (IHMSA silhouete) at 50 yards.
The one thing I can tell you, is to make your own reloads, since all the comercial loads aren't for this size of barrel. For the other side, shoot hot ammo on this "snubbie" is one of the most beautiful and pleasured things in the world ! My friends called my snubbie "NAPALM".
About ballistics, it's about 10% less than the 4".
Hope it helps.
Andre Tiba - Brazil