If the idea is to “trade the $US”, then how that trade gets implemented depends on which vehicles are chosen. E.g., to go “long the $US” can be done two ways: directly (by buying an ETF that goes long the $US), or indirectly (by shorting the Euro or the Yen). The same is true of the reverse trade. The $US can be shorted by going long the Euro or long the Yen.
Additionally, there is “leveraging” to consider. The $US dollar long/short pair, UUP/UDN, can be leveraged to 2x/-2x, with the mutual fund pair, RYSBX/RYWBX, or with the Euro long/short pair, ULE/EUO or the Yen long/short pair, YCL/YCS. Lastly, there is “liquidity” to consider. Anything but UUP is thin and trades on average daily volumes under 100k share. The exceptions, of course, are RYSBX and RYWBX, which don’t report volume and which only price EOD.
I think the $US dollar needs to be tracked and traded. How that trade is implemented is where things get tricky. Right now, the $US dollar is the least dirty shirt in the fiat currency laundry basket, and it is appreciating against all currencies except the ruble and the real. But as the world’s economies continue to “dedollarize”, I expect the $US dollar roll over and depreciate against currencies pegged to gold and/or a commodities basket.