I like to use math to help me "visualize" roughly what different types of equipment will require for curve radius. It helps because I spent many years in O and HO and I sort of have a feel for what will work and what won't. Example, small engines and 40-50ft rolling stock will typically run on 18" in HO. The simple formula ((18*87)/64)=24.47 will get you in the general ballpark of what radius you will need.
*Important point - Radius requirements don't have anything to do with Scale vs. High Rail flanges. With the exception that most High Rail engines are built with compromises to help them negotiate tighter curves. Example: Blind drivers on steam engines and swinging pilots on diesels. You could make the same modifications to a scale engine and it would operate on the same curve radius as its High Rail counterpart. 2 Rail O scalers have been doing this for many decades. Conversely, if you have a High Rail engine with all drivers flanged or fixed pilots and body mounted couplers it will take the same wider curve radius that a scale engine would need. The size of the wheel flange doesn't dictate radius...
This is my biggest pet peeve about S. Unless you want to build hybrid closed frog turnouts (Link to turnout how to) with a rail large enough for High Rail (Code .125 and up) you will need to choose one or the other.
I personally chose scale. I wanted a larger version of HO, not a smaller version of 3 rail O. I don't run any American Flyer either. I also like the look of code 83 and 100 rail much better. Not to mention that there are very few High Rail turnouts available. There is a much wider variety in scale.
Think about what type of equipment you want, what kind of railroad you want, research what is available and make your decision. Example: My favorite diesel is the EMD GP30. It was only done once and that was in S scale brass. Things like that helped me make up my mind.
Good luck.
