The S&W 64 is one of the finest revolvers made. If you should get one that shoots right on point of aim you should keep it forever. They are very, very accurate handguns (with the right ammo, of course). My old service revolver (model 10, the blued version of the 64) is my favorite handgun (and I own a bunch...). On occasion I would use it for testing ammo accuracy (via a Ransom Rest) at the Firearms & Tactics Section outdoor range. With the right ammo it would put all it's rounds in less than a 1" circle at 25 yards. Really.
Round butt or square? Can't go wrong with either one. I own, among other revolvers, a 4" model 19 with a round butt and a couple of model 10s with square. Love 'em all.
Go buy one. Trust me on this one.