There is no one right answer. You take into consideration color, cost, and how easily you want it to fit.
If you want metallized, you'll probably end up going for premade eyeglasses/visors/whatever, and then bondo'ing the helmet to make it fit the visor. Fast but limited on choices.
If you want a good fit but don't mind it being a flat color or clear, you will get good results making your own vacuum formed pieces, but that is difficult and time consuming.