The pancake is f/2.8 and for my personal style with photography I like shooting wide open. So I like having the option of using a larger aperture of 1.8 vs the 2.8. Just personal preference. The 18-55 is def fine for the long exposure shots I want the 35 for day time shooting and low light situations where I can go to f/1.8 and just bump up the iso and not do an extended shutter speed.
oh and the 40mm pancake lens is for Canon I have a Nikon at the moment lol.