Why on earth would we use gold?!
I say chips.
Well in fallout 2 the currency was gold, people figured out how to mine gold and turn it into a coin in which was used to trade all across Nothern California and parts of Northern Nevada. Some would argue that gold should be the currency because thats what fallout 2 left us off with, the capital wasteland is too far away to use their currency caps, while the towns in fallout 2 are way closer to southern Nevada, or New Vegas. Plus fallout 2 left of us with a plot that involved the NCR wanting to control the gold mining town redding which was having a conflict between choosing sides with the New Reno mob or the NCR. But at least people did figure out how to mine gold in the wastelands, gold mining is nothing new ancient people have been mining gold for centuries, so it is possible that people would figure out how to mine gold and smelt into coins. People have mining and smelting gold for centuries way before modern technology came along.
Anyways I think gold, chips, and caps are the best choices and the currencies I'd like to see in the game. Anything else would not fit in the game.