610's do indeed use moon clips. Shooting .40 S&W through a 610 is akin to shooting .38 specials through a .357 magnum.
As far as barrel lengths, I'll be the one to go against the grain here. I'm looking at the 6 1/2" barrel length. This will primarily be a range gun for me, the added sight radius when shooting irons is a plus. If I decide to use it for hunting, it'll likely get scoped and carried in a chest rig.
Not to say the 4" wouldn't work. Truth be told don't be surprised if I end up with one of those as well. I'm kind of weak-willed that way...
Now if S&W decided to release a 610 with a 5" barrel, I'd be at the head of the line!