My preference would be a .357 so you have a double-duty gun if you ever want to use it for something more than targets (you never know). And my preference for a .357 would be a 4" or 6" Model 66. For range work by a young or slightly-built shooter, they are less bulky than an L-frame gun.
My favorite target ".38" is a 6" Model 66-2 with target trigger, target hammer and a Millett rear sight. I would love to have the integral red ramp front sight replaced with a pinned Patridge but I hate to permanently alter an out-of-production gun.
Ed