Which war would harm the warriors more? TRADE or CURRENCY? Why?

Well ... currency manipulation is a type of trade war.

China vs USA example:

USA imposes tariffs on Chinese goods.
The cost of Chinese goods increases for Americans.  American government gains revenue from tariff "tax".
China reduces the value of their currency, relative to the dollar.
The cost of Chinese goods decreases for Americans.  The cost of American goods increase for Chinese.