Is it better to have soft spring in front or in the back

Like all setup changes it depends on what your car is doing. If you WANT more front grip then run softer front and stiffer rear. For more rear grip run softer rears and stiffer fronts. There are just too many ways of getting the same thing with cars as complex as RC cars are. I can dial in oversteer to different degrees in my car with 20 different chasis and suspension changes. Remember the "ballast" I had on my car? Also a ton of rear camber will add to the oversteer and so do the spring rubbers I added in the back.