When "stick" is enabled objects appear to become one object, but really they act separately, however because of this if you have three objects stuck together, there will be a wired glitch where momentum is gained even though it should be conservered.

The reason you would use the runge-kutta method is so you can approximate the new location of the planets better. The book "Physics for game programmers" by Grant Palmer has a really good explaination for why you should use the Runge-Kutta method for approximation in a scenario like this. I am going to be using this in my own orbital simulation soon.