I'm another who prefers the 6 shot 617, but that's mainly because I seem to have trouble counting to more than 6. Yesterday was fairly typical, had one unfired round on the first 3 cylinder loads with my 620 because it took that long to remember to count to 7. That aside, you'll want to look into the rules for any competition that you're looking into taking part in, some venues will limit your load to just 6 for a revolver and indexing a partial load will slow you down on a reload.
As for barrel length choices, that again will depend on the particulars of the competition. In terms of accuracy, the 4 inch barrel should in theory be the most accurate. As for why, it's shorter length will result in a higher coefficient of stiffness, which means that it will deflect less in response to a bullet transitting the barrel. However that advantage will be so slight with the 22 caliber that I doubt that it's even measurable when compared to the 6 inch barrel. From a really rigid machine rest, the advantage might be detectable when compared to the 8 3/8 inch barrel. Point is, if you're allowed to use optical sights and don't need that slight gain in velocity from the longer barrels the 4 inch version is the best choice. It you have to shoot with iron sights, the longer barrels are a gain due to the longer sight radius. In addition, if you're shooting the mini steel silohuettes for the 22LR, the extra oomph of the longer barrel will yield a gain in knock down ability.
Basically, if you're serious about shooting the 617 in competition, you need to sit down and look over the rules and let that drive your decision on barrel length or round count. If you want something to train with or for just plain fun, choose whatever floats your boat because you can't go wrong with any version. Personally for a pure plinking or training gun, I'd go with a 6 shot 4 inch 617, but those are NOT easy to find. Second choice would be the new 10 shot 4 inch. However, like some others, I would avoid the aluminum cylinder simply because it won't last as long as a steel cylinder.
PS; I have the 6 inch 6 shooter and it is rather muzzle heavy. However, it was a bargain at a dealer that mainly deals in 9mm Highpoints, so I snapped it up. While I would have rather had the 4 inch 6 shooter I won't sell the one I have, it's just too much fun.