Gold is going to be somewhat harder to spend.
"The Serious Organised Crime Agency claimed that "90% of all €500 notes sold in the UK are in the hands of organised crime", revealed during an eight-month analysis." [1]
€500 weighs 1.12g, but is worth $1.11, so US$1M in €500s is 2kg. A very decent saving.
Of course, the UK doesn't use the Euro, so it would be interesting to see the same analysis done in several different Eurozone countries.
The bit explicitly mentioned that US$1M fits neatly inside one briefcase when using US$100 bills, but it would take three briefcases with US$20 bills.
This may be true, but I'm a little wary of the argument now, as the drug trade, counterfeiting, and money laundering line was used when the US$1000 and CAN$1000 notes were retired, and they are worth less now than they were then. Loss of high denomination banknotes has not impeded the business of the underworld, but loss of cash transactions means that everyone in legit business now has to pay a tax to Visa and Mastercard for acting as an intermediary in payments processing.
A pile of $100 bills is $100/g, or $45400/lb. $50 bills are $50/g, or $22700/lb. A $20 bill is about $20/g, or $9080/lb. Gold is $38.82/g, or $17607.88/lb. Clearly, if you eliminate the US$100, cash movers switch to the US$50. If you eliminate that, they switch to gold, not the US$20.
US$1M in $100 bills is 22 pounds (10 kg) of cash. In US$50 bills, it is 44 pounds (20 kg). In gold, it is 57 pounds (26 kg). In US$20 bills, it is 110 pounds (50 kg).
Someone in the monetary bureaucracy might want to reconsider the unintended consequences of the black market drug trade switching to gold as its preferred currency rather than paper money.