1) The .357 magnum is very barrel length sensitive in snubbies. The 3" will give better ballistics. You cannot make generalities about barrel length vs velocity. The .357 uses slower powders which actually work much better in a 4-6" or longer barrel (say 20" in my carbine). .45 or 9mm is much less sensitive to barrel length. There is little advantage in going to a longer barrel and carbines offer not much extra at all because these calibers use fast burn powders and are more efficient. For a magnum revolver round, longer barrels are definitely better.
2) The extra sight radius makes the longer barrel come on target quicker on the draw and easier to sight. This translates into quicker times and better accuracy. The gun's intrinsic accuracy isn't any better, it's just easier to shoot well with the longer sight radius. I have a 3" .38 and have owned several 2" .38s and I can tell you this for a fact. The SP is no pocket gun anyway, so I fail to see the use in the shorter barrel. You can carry the longer barrel gun IWB just as easily and if it slows the draw any at all, the longer sight radius will make it faster to acquire the sights on target.
For me, it's a no brainer in the SP, go for the longer barrel. I want one right now for some reason, maybe the strength of the SP combined with the power of the .357 magnum, but I'd like to own one and have been thinking of buying one if I could get a good price on one. It'll be the 3" version, which I think is actually 3 1/4 or 3 1/6" or something. It could even double as an outdoor hiking gun with heavy loads involving heavy hard cast bullets. For that duty, I'd definitely want the longer barrel.
People say accuracy is irrelevant at "defense ranges" and only practice at 7 yards.
Myself, if the gun can regularly knock down 6" plates off hand at 25 yards if I do my part, I don't think it's accurate enough. That's just me I guess, but I've always been a sucker for an accurate gun, probably my outdoor/hunting background. I like to hit what I shoot at.
What I like about the SP over K frames and especially a pig like the L frame or GP100 is it's lighter and more compact for CCW. I would NOT carry any GP100 IWB more'n 5 minutes, myself. It's a heavy pig. It's a good outdoor gun, good home defense gun, fun range gun, but it is definitely NOT a good choice for CCW IMHO.