Dragonite with Dragon Tail Hurricane or Dragon B with Hurricane

I think the higher IV one will do better for you in the long run. It will likely have over 100 CP on your 70% one. Also I believe Dragontail is better DPS than Dragon Breath.

If you prefer Dragon Breath then power that one up. You probably won't notice the difference in IVs when actually using it in battles and small differences in CP among same-species Pokemon of the same level are not really relevant anymore with gym defence being changed.

Dragon Tail does have slightly more DPS and EPS but it's a game so do what makes you enjoy it. If you don't enjoy the slowness of it, then Dragon Breath is still a great move.

Dragon Breath isn't a bad move or strictly worse, it does have better dodging, so it's not like there is absolutely no reason whatsoever to power up a DB Nite over a DT one.