That one line is "torp boats" and the other "gun" is a bit of a misnomer. Currently you have the main line ending in the Shima who has torpedoes akimbo and a sub branch that ends in Akizuki which happens to be a gunslinger. Right now the tiers VI and VII are setup where the ships that are more geared towards stealthy torping are in the Akizuki line while the more aggressive Special Types are leading to the Shima. Word on the grapevine is WG is going to be doing some tweaks to them in the near future, hopefully that means restructuring the trees more sensibly and not nerfs. (PLZ not nerfs, how better turret traverse instead? k thanx)
Historically and gameplay wise Hatsuharu>Shiratsuyu> Kagerō>Yūgumo>Shimakaze would be the logical progression of destroyers. These all represent an unbroken line of development of the IJN's torpedo destroyer concept with each a refinement of the predecessor. Having Fubuki>Akatsuki>Akizuki also represents a better progression. Fubuki and Akatsuki both are Special Type Destroyers, fast heavy fleet units built to operate with heavy cruisers, while the Akizuki was conceived to be a fleet escort/AA boat.
Though take this with a grain of salt, none of these IJN DDs sans the Akizuki are real gunboats. You aren't going to be reliably winning knife fights with the USN with just guns. That said overall the guns on all the IJN cans are a little better and they can be integrated into your gameplay, especially at mid to long ranges were they prove accurate and the slow traverse is mitigated. I've been working out a good 20-30k in gunfire in the Shiratsuyu even though Akatsuki has a better gun load out and arrangement.
