The difference is that you can devide bitcoins into smaller units. And how is a currency that you can print at will a good thing?
You can do the same with that silver backed dollar, half-dollar , quarter, dimes, nickles, cents.
The difference is back then, it appeared the US believed in a limited Currency, something where there was only so much of X behind it. Just as now there are only so many bitcoins out there.
Bitcoin today is pretty much based on that silver certificate (granted you have to remove the technology differences of 100 years)
Silver was limited in amount available
Bitcoin is limited in amount available
Both are divisible down to smaller units
Both were not able to be printed off at will