Consider that no major currency will be worthless any time soon. I would hold the currency that matches your future expenses. If you plan to live in the US most of your life and spend most of your money in US dollars, hold dollars. This way currency fluctuations impact you the least (because savings are in the same currency as expenses). Of course you could (partially) hedge against a USD dollar decline, but in that case I would choose a basket of major currencies and not a single one.
You can consider holding HKD. It's pegged to the USD and thus you have no exchange rate exposure under normal circumstances. However in the extremely unlikely case that the USD does fall apart the HK government has the option to unpeg the currency and likely scenario is that it would rise instead of fall in value against the USD. So I think there is some good downside protection there without the exchange rate risk. Of course there is always risk of HKD falling apart instead of USD and afterall it's a tiny economy. However things are relatively quiet and there is relatively little problem here except for some LEH CDD papers holders who got scammed into thinking they're buying "minibonds" and are now protesting on the street.
I did a little bit of hedging using Merk's funds - you can check their fund holdings. Not recommending them, just for reference. http://www.merkfund.com/fund/mhcf/index.new.html http://www.merkfund.com/fund/macf/index.html