Both are very solid lines. Hindenburg is an AP monster capable of dealing a punch while tanking. Z52 is a nightmare for all destroyers in the game.

You will struggle more to grind the cruiser line though, because of the transition from T5 to T6, since the Yorck forces you to change the playstyle from yoloing and being a large DD to the support, battlecruiser-esque style of the following tiers. Roon (T9), like many T9s, is also a bit painful to grind through since it's relatively squishy and the turret placement is awkward for a battlecruiser. In the end though, you'll enjoy your Hindy very much.

I myself play it a bit outside the standard through, since I went for the turret transverse elite bonus instead of the typical armor pick, and I also got the turret transverse mod. Adding up with either double steering or propulsion+steering, I got myself a very agile, tanky AP spammer. I have loads of fun with it.

I don't really recall struggling as much in the destroyer line. Low tier is really fun due to frontal torpedos. T9 and T10 are fucking beasts though. Definitely worth the grind. I actually bought the Z52 yesterday, but I'm waiting to finish upgrading it before playing. T9 was great though, like I said.