I haven't responded to you because I didn't want to speculate. However, I personally see little use for the devs implementing the galaxy model in the simulation "Galaxies Collision" when they are already developing a better model, which uses dark matter to achieve perhaps a more accurate simulation.
I believe the "better looking" model in the pre-made simulation is the older one, and the new model is being worked on. It doesn't look as good yet, but I would seek to believe they have chosen the latter one to keep around in the "Add" tab because it is part of their foundation for the new model. Perhaps, there's something the models in the "Galaxies Collision" simulation that can't do where the new ones will be able to do. They don't need to keep the old model around when the new one is already being developed and on its way.