both excellent choices, both very (equally) accurate
I own both (in the older vintage style), the 4" 63 and the 6" model 17
one is not 'better" than the other
The k-frame 17 has a much better DA trigger
(that is generally well known and acknowledged, K vs. J attributes, not an isolated opinion)
neither is more accurate than the other, the larger frame size in equal length barrel may be easier to shoot more accurately
a k-frame with 6" barrel likely easier to shoot more accurately also, longer sight radius being more user friendly
the smaller J frame handier to tote around, if you carry in the woods or whatever
as a "serious" target gun, I would favor the 6" k-17, likewise for rapid fire DA practice
but the 4" 63 as a remarkably accurate fun gun, that can "shoot better than you can"
PS
I do believe the 617s are actually L frames not K frames, most 617s are full underlug barrels, to clone the 38/357 L/N frames
the 17 is a true k-frame, of course
(might as well say that now, before somebody 'corrects' me, you know)
I like the traditional K better, half underlug, but it doesn't really matter to most folks
but it really depends on what best fits and feels "just right" in your hand