What era are you interested in? Code 83 was designed to simulate modern mainline rail. In my opinion, the best thing you can do is show a size difference between mainlines and sidings. Use 83 for mainline, 70 for sidings. If you are really daring, use code 55 for industrial tracks. If you are doing pre-wwII, code 70 with code 55 sidings would look better.
Some things to consider: Older european made equipment, such as Rivarossi, older IHC, or AHM, won't run on less than code 83. Also, the smaller the rail code, the less forgiving the trackwork seems to be. A small bump may get it out of gauge, and extra care must be taken to make sure things are smooth and level when using smaller rail sizes.