Having both a 6" 586 and a 6" 629 Classic DX (both w/ full underlugs, needless to say) I at first figured that a 6" 617 would be the logical choice due to my being accustomed to the weight distribution factor.
However, my wife2B messed all that up by giving me a 4" 617.
My own practical experience has been that the 4-incher feels to me like the balance is more to the center of things. In reference to rimfires where muzzle flip isn't a consideration, I'm diggin' it!
Regarding accuracy discrepencies due to the shorter sight radius, I prob'ly don't shoot well enough to be able to tell the difference.
I'm used to shooting .22's with my 8&3/8" mod. 17 no-dash and the difference in balance is obviously quite noticeable, not to mention the LOOONG sight radius. More accurate? In the right hands, quite possible if not probable. But heck, if I'm really bent on trying for 10 shots in the same hole @ 50 yds. or whatever with the 4" 617, I can always put a red dot on it to up the odds a good bit. The most important thing is to know your gun.
I'd guess that your best bet would be to rent one of each at a range if you can, remembering the ammo-finicky-ness of the beast in question, and you'll know the pros and cons of each for yourself pretty quickly. It should be pretty easy to choose by then, I'd think.
Whichever you decide upon, be ready for some serious FUN!!!!!