Being the happy posessor of a 9mm Luger with a barrel length of 5 inches, I feel like weighing in here.
Esthetically, 5" is definitely the proper length for a Luger barrel. The 4 3/4" .30s come close, but they are a bit too skinny.
In the hand, the 5" feels much better balanced than either the 4" or the 6". In comparison to the other two, a 6" Navy somehow gives the impression of being a very much larger gun overall.
The effect of the sight radius advantage of the longer barrels is considerably dependant on the skill of the shooter, and somewhat on the individual gun. My 4" S/42 is extraordinarily accurate. In my hands, the 5" is no less accurate, though certainly no more (Thor's test target for the 5" reveals the limitation of my own abilities, however [img]frown.gif[/img] ). I have only shot my Navy once, so I can draw no conclusions. I will note that my Artillery is more accurate than any of the other guns, longer barrels do have an inherent advantage.
After an hour's shooting of the longer barrel lengths, the 4" seems strangely compact in comparison. Now, here's the odd thing. After all this, the standard 4" barrel seems the most "weapon-like", and indeed, the proper length barrel for a Luger. The 5" has better balance and "look", the 4" feels more like part of my hand.
Ymmv. I don't know if this has helped answer your question...
(Yes one of these days I will actually manage decent pictures of this guy)
--Dwight