Whatever works for everybody.
I'm running an 18 front and a 40 rear on my 03 America. At an indicated 60mph it's turning right around 3000 rpm. I don't miss the low end pull because I ride solo and don't race anyone. Since recently making this change, I've gotten 53mpg and 58mpg on the last 2 gas fills.
It's all in what makes you happy. The 16 will definitely give you max grunt off the line....
