I also wanted a model 617 as a sub-caliber practice gun and was worried about the differences in "feel" between a 6-shot and 10-shot versions. I ended up getting a 10-shot as I would also be using it for handgun hunting and plinking, and I didn't want the capacity limitation for those pursuits. There is a difference in the DA pull, but it's close enough that only the most dedicated DA users would notice it.
So, if ALL you'll be using it for is sub-caliber practice for your other revolver(s), then getting a 6-shooter MIGHT make more sense. If you want to use it for anything else, then the 10-shot gun offers a higher capacity without any other negatives (other than a quicker drain on your ammo supply).
5 shots, 25 yards, rested, shooting DA; best group with cheap ammo, so far: