Okay, lets start with Single Strike form.
Urshifu Single-Strike form's signature move is Wicked Blow. It's slightly more powerful than Urshifu Rapid-Strike's signature move, Surging Strikes. Urshifu Single-Strike has 2 weaknesses and one 4x from fairy. It does have an immunity on Psychic type but a 4x weakness from Fairy is not good. However, Most people use it because its signature move, Wicked Blow has massive power and can easily go first and sweep. It has a weakness from Fighting type which is also not good, but its other good points outweigh the bad points. Urshifu Single Strike with only its Fighting and Dark type, is super-effective on 422 Pokemons.
Next, the Rapid Strike form.
Urshifu Rapid-Strike form's signature move is Surging Strikes. As I said, it is less power than Wicked Blow but is still very powerful. When Urshifu Rapid-Strike Dynamaxes and uses Max Geyser, it will summon rain to make its water type moves even stronger. When it G-maxes, however, G-max Rapid Flow does not summon rain although strikes through protection. With only Water and Fighting type, Urshifu Rapid Strike form only is super-effective on 384 Pokemons, less than Urshifu Single-Strike form.
In all, I would choose Urshifu Single Strike form instead of Rapid Strike. It is super-effective on more Pokemons and you can just switch out when you're in danger. You could always choose the Rapid Strike form if you want. The Tower of Darkness is also essentially easier than the Tower of Water because your Kubfu is super-effective on the Tower of Darkness. I hope I helped!