Many modellers contend that, once it is all nicely ballasted, the ballast and rails weathered, and a big steamer placed atop them, there is little difference in images between the Code 100 rails and the Code 83, for example. I disagree. I used Code 100 because it is readily available locally. Now, a couple of years later, I can definitely tell the difference. Code 100 scales out to about 170 lb/yard rail, somewhat larger than any every used for mainline service in N. America. The might PRR steamers used 150 or 155 lb/yd on their mains.
On the other hand, images depicting Code 83 and Code 70 track look much better in photographs. If you are certain that you'll never want to post images showcasing your modelling, Code 100 is the way to go for many people who want it quickly.