R is the speed rating of the tire, which is rated for up to 108 mph. Stock tire has an H rating which is good to 130 mph. Personally, I'd stick with the H rated tires.
I don't know about the America, but on my Speedmaster the 110/90 - 18 is as tall as I'd want to run. There's about 1/4" clearance between the tire and the inside of the front fender, where before with the stock tire there was more like 3/4" of clearance.
Various aspect ratios of some of the tires in question on this thread:
120/80 = 96mm or 3.78 inches(sidewall heighth)
110/90 = 99mm or 3.90 inches
110/80 = 88mm or 3.46 inches
